Only as long as it needs to, of course. The best way to gauge this is by the total number of sets you do. I first learned this years ago from famed Australian strength coach Ian King, and I find it still holds true today. The advice: Do 12 to 25 sets per workout. That is, add up the sets you perform for each exercise and the total should fall within this range. (Not including your warm-up.) So if you're using lengthy rest periods, your workout will take longer, and if you're using shorter rest periods, you'll finish faster. Beginners will probably find that 12 sets are plenty, while experienced lifters may be able to handle the upper end of the range. This total-set rule isn't set in stone, of course, but it works very well for building muscle and losing fat. For most people, doing more work than this in a single workout results in rapidly diminishing returns on their time investment. It also increases the time your muscles need to recover between your bouts of exercise. If you ignore this important factor, you can wind up overstressing your body, which slows your results.
How many sets of an exercise should I do?
A good rule of thumb: Do as many sets as you need to perform at least 25 repetitions for a muscle group. So if you're planning to do five repetitions of an exercise, you'd do five sets of that movement. If you're doing 15 reps, you'd only need to do two sets. In other words, the more reps you do of an exercise, the fewer sets you need to perform. And vice versa. This helps keep your muscles under tension for an appropriate amount of time, no matter what rep range you're using.
If you're in good enough shape, you can certainly do more than 25 reps per muscle group, but cap your output at 50. For example, a common bodybuilding recommendation is to do three sets of 10 of three or four different exercises for one muscle group. Do the math, and you'll find that's up to 120 total reps for the working muscles. Trouble is, if you can perform even close to 100 repetitions for any muscle group, you're not working hard enough. Think of it this way: The harder you train, the less time you'll be able to sustain that level of effort. For example, many people can run for an hour if they jog slowly, but you'd be hard-pressed to find anyone who could do high-intensity sprints—without a major decrease in performance—for that period of time. And once performance starts to decline, you've achieved most of the benefits you can for that muscle group. So why waste your time?
How many days a week should I lift?