To build strength, should I lift weights three times a week for one hour or six times a week for half an hour? Does it make any difference?
The number of sessions per week depends on what muscle groups and movement patterns you train in each of those sessions. To build strength (your stated goal), there is no serious controversy about how to do that best. You will want to warm up to and train with relatively heavy loads (80–100% of your one-rep maximum), for generally multiple (3–6) sets of relatively low repetitions per set (generally the 1–6 range). You also want to emphasize basic, compound (multi-joint) lifts, such as squats, deadlifts, power cleans, presses, rows, and chins. It is impossible to perform sufficiently intense (heavy) and voluminous (total reps and loads lifted) training to develop strength over the long term (for months on end) without allowing your body to recover and adapt from one session to the next. This is critical - the additional strength is an adaptation by your body that takes time and rest for the requisite physiological changes to occur. General rule - it should be at least 48-96 hours between successive sessions training the same muscle groups and patterns. The harder you train in a given session, the more time it will take to recover before you can train hard enough to build strength in a subsequent session. In other words, there should be at least two to four days between workouts that stress the same muscles and movement patterns - I.e., if you strength train your legs on Day 1, you should not train them again until Day 3, 4, or 5. So three times per week, for the same muscles/movements, is pretty much the absolute max frequency for serious strength training, and hitting the same patterns hard twice a week is much more commonly recommended and practiced (for example, you might bench heavy one day and overhead press heavy several days later, within the same week - since both lifts use many of the same muscles in the same type of motion, namely pressing/pushing). If you train the same muscles and movements three times per week, one or two of those workouts will have to be a "light" or "moderate" session - because you will not be able to recover fast enough to train in the actual (very hard) strength-building range and do it three times per week on just 48 hours rest. Some very, very good programs and schedules for you to follow would include popular and proven one such as Starting Strength, Wender's 5–3–1 system, Stronglifts 5x5, and so on. You will be able to do those programs, if you are time efficient, on a 3 x over week for one hour basis. If you really want to do 30 minutes six times per week, that can work too - just alternate between lower-body (leg, hip, and spine) exercises on even days, and upper body (push/pull) type exercises on odd days - and you won't have enough time to do more than basically one major lift each day - but it can certainly work,