How often do native Arabic speakers speak modern standard Arabic?
Almost never. I have lived in Lebanon and Syria (before the war started), for quite some time and I learned to speak Arabic and I'm pretty fluent in it now.
Modern Standard Arabic, aka. official Arabic, is only used formally. This means only in publication, essays, letters, formal speeches, etc.
But conversationally, it's never used. If you're a tourist and visit an Arabic country and speak modern standard Arabic, they will forgive you and wouldn't make a fuss about it and will go on with you. But if a native started speaking modern standard Arabic he would get ridiculed extensively (his friends would at first think he's doing a skit, but strangers like shopkeepers and clerks will not understand what's going on and will perhaps be weirded out). Speaking MSA in normal everyday situations is the equivalent of speaking Shakespearean English in modern day America or UK, it's just not right.
If you, as a tourist, speak modern standard Arabic to natives, they will understand you and will be able to aptly answer you. But between each other they will speak regional Arabic.
For the average Arab, speaking modern standard Arabic (MSA) is almost null, but writing MSA often exceeds 50%, I will explain in detail what I mean.
For me, I don't use MSA to speak at all, but in rare cases, if I was asked to translate something from English into Arabic, I would like to translate it in MSA, and if the translation eluded me, I try to translate it in the Syrian dialect. Sometimes, I speak MSA as a joke with my wife and my friends. Most of us feel awkward when speaking MSA because we are not used to it and rarely use it in our conversations and often make grammatical and verbal mistakes.
I remember one time I met a friend of my cousin. She was a Chinese student who learns MSA in Syria. She did not understand the Syrian dialect and therefore we had to speak with her in MSA. I remember how my relatives and I were mocking each other about how we used MSA.
As for writing, the Arabs sometimes use the MSA, for example, in writing a poem or a general notion or even a post on Facebook, not to mention using MSA in universities, schools, and government departments. However, all chats in social media, in general, are in Arabic dialects.
With regard to listening and reading, it happens a lot through print, audio-visual media and there is no difficulty in understanding the modern standard Arabic for all age groups.
For the elite class of Arab politicians, thinkers, and intellectuals, they often speak in MSA whether in the media or in public festivals. They're a lot more proficient in MSA than the average person.
By the way, ISIS uses classical Arabic, an old version of Arabic used in the first period of Islam. It is more complex and difficult than MSA.
My friend told me a story about a member of ISIS who spoke classical Arabic during his arrest. It happened after a painful blow that ISIS has recently received from the Syrian Army, in which many of its members were killed.
The ISIS man was running away when he saw a villager on a motorcycle,
ISIS man: هل انا في ارض الخلافة ام في ارض النظام الكافر؟
(Am I in the land of the caliphate or in the land of the infidel regime?)
The villager: انت في ارض الخلافة
(You are in the land of the caliphate)
ISIS man: اذاً, اريد طعاما
(So, I want food)
The villager: حسنا اصعد خلفي على دراجتي النارية و سأعطيك ما تريد
(All right, get on my motorcycle and I'll give you what you want)
The villager then handed him over to the Syrian army and asked them to feed him.
I'm surprised of those that say ‘almost never'. While we don't really use it a lot, it's far from never.
There is obviously the formal use: official speeches by government officials, official documents, reports..etc.
Then there is the religious use: scholars, the Khutba, fatawa, even the religious discussions on TV.
Schooling and education: officially, all education is in MSA. While the teacher in the class may speak a dialect, the books, the reports, the essays, the homework...etc. is all in MSA.
TV: While some programmes are in dialect, and recent advertisements are in dialects, quite a lot is in MSA - arguably most. The news, any news related reports and/or discussions, any religious or religion related programme (even the ones for Christians, unless it's in another language such as Syriac or Coptic), children's programmes, educational programmes, documentaries. Subtitles are always in MSA.
The written word: If you are going to read anything, it's going to be in MSA. Books (including fiction), newspapers, magazines, written ads, the tags on the products in the market, business correspondence, birthday cards, technical manuals, notices, medical reports, road signage...etc.
Art and literature: apart from the written word, any words used in calligraphy or other artwork are mostly in MSA (I've personally never seen any in dialect, but it's a possibility), and while there is a lot of poetry in dialects, most poetry is in MSA.
Songs: there are many songs sung in MSA or Classical Arabic (Arabs actually don't really differentiate between the two, both are called fusHa). Some of the most famous and popular singers have done that (Umm Kalthoum, Abdul Wahaab, Fairouz, Majda Al Roomi, Milhim Barakaat, Abdul Haleem Hafith, Kathim Al Saher, and more recently, Maher Zain and Shahad Barmda - among others). Of course, I'm not talking about religious songs or songs about patriotism (which are mostly in fusha), I'm talking about everyday ordinary love songs.
Also, some traditional ‘mawaal' (a part sung without music at the beginning of the song) are in fusHa, mostly verses of poetry.
Other than that, we do actually use it in everyday life. Of course, we are not going to have a discussion in MSA, but we quote words from the Quran and Sunna, we quote poetry, some proverbs are in MSA, and we sometimes use MSA phrases in daily speech. They are so ingrained in the language that we don't even notice that they are MSA or Classical Arabic. As an example (phrases I have personally heard being used): في أمان الله - أخجلتم تواضعنا - السلام عليكم - وعليكم السلام - على أهلها جنت براقش - الصديق وقت الضيق and others.
This, of course, and don't forget words that are borrowed from MSA and used in our daily lives.
So, we actually use more MSA in our daily lives than we think we do. I'd say that this is hardly ‘almost never'!
Saying that "Arabs do not know or even speak Arabic at all" strikes me as nonsense. Conversationally, we [native Arabic speakers] never do; sometimes, children who listen to the MSA in animation movies would speak in MSA and that sounds extremely funny; and also when foreigners speak in MSA, you should expect the average Arabic speaker to be really gobsmacked at first before he makes his mind and tries to reply in MSA; in such situations, it might sound kind of embarrassing that foreigners speak your alleged native tongue better than you do. (In fact, the native tongue of every ‘Native' Arabic speaker is his own dialect because that's what he first learns from his parents; the MSA is taught later on at school.)
In fact, even though most native Arabic speakers can understand and speak the MSA, they are not accustomed to speaking it; foreigners who learn Arabic often do better when speaking the MSA because they are accustomed to it, unlike native speakers who rarely, if not never, use it in conversations.
However, we use the MSA in schools-in books, homework, exams, presentations, documentaries, etc.-even though teachers often use their dialect when they explain. In universities MSA is used more often than in schools-yet not to a great extent.
When I was a 2nd-grader in Libya-and I'm an Egyptian myself-we used to have an Egyptian Arabic teacher who used to speak in Egyptian dialect or more often in the MSA during our classes, because it was sensible that everybody will understand either both especially that our class was a "multi-dialectical" one-there were Libyans, Egyptians, Moroccans, and even Syrians. Ironically, no student of us used to answer [the teacher] in MSA, but rather in his own dialect, and our teacher used to understand them all.
Only in formal situations such as news broadcasts, conferences, public speeches, and in the media.
Otherwise, they either use the dialects that are applicable to them or their native languages alongside the said dialects.
This is especially true for North Africa where the various Berber and Nubian languages are still spoken.
In some countries, they would either use French or English due to British and French influence coming from past colonialism and mandates. This is especially true for the Maghreb and the Levant.
Often, the speakers would have their dialects mixed with elements of the two languages to varying degrees.
What must also be noted is the existence of an interlanguage that has F'usha elements mixed in with dialectal elements and visa-versa.
This form of Arabic is frequently heard many formal and informal settings, most notably in the classroom and during interviews.
This speech register remains poorly documented unfortunately, which is a pity as it offers a crucial window into the diglossic situation in the Arab World.
I view it as more of a continuum. If two native speakers come across each other walking down the street they will speak colloquially. Now if the topic of discussion shifts from the weather or the price of tomatoes to politics or religion, the language will shift slightly to be more conforming to modern standard.
This will also happen if you have two native speakers with very different dialects. For example, an Iraqi and a Moroccan will likely speak in a manner more conforming to MSA (though not really all they way there) rather than try to understand each others dialects.
All of this also very much depends on the education level of the two speakers, which country they studied in, and what educational track they were in (sciences vs. liberal arts). Liberal arts tracked people generally have stronger Arabic language skills and are better able to converse in it.