How long of not working out will noticeable decreases in strength/muscle mass start to occur?
It depends on what you define by inactivity and what does your top workout looks like.
If you are a beginner-intermediate weightlifter, and your daily life includes walking, running, lifting moderate weight like bags/babies/water bottles/whatever, and you, say, have sex on top regularly, then you probably loose nothing for a very long time.
If you are a pro, and your daily life is sitting in the office and eating popcorn on sofa, then in means that your muscles will be performing at like 5% of their maximum possible strength, and you will start loosing muscle and strength in one week.
If you are world champion, and the next day you quit the gym you'll take the job at the space station under zero gravity, you'll loose a lot of strength in days. If fact, if you are a world champion, even having daily warming up exercises such as push-ups and jogging will make you loose strength.
So a lot depends of what does your "not-working out" looks like? Your top-hardest workout possible is 99% of your real capabilities. How much percent is your "not-working out"? The larger is the difference between it and your top, the faster you loose strength.