At least two. This amount has been shown to provide many of the health benefits attributed to resistance training. So consider that the minimum. Ideally, though, you'll want to hit the weights three or four days a week, with either total-body workouts or an "upper-lower split" approach. I'll explain each.
Total-body workouts are just what they sound like. You work your entire body each workout. Then you rest a day, and repeat. There's a scientific rationale for this. In multiple studies over the past decade, researchers at the University of Texas Medical Branch in Galveston have reported that muscle protein synthesis—a marker of muscle repair—is elevated for up to 48 hours after a resistance training session. So if you work out on Monday at 7 p.m., your body is in muscle-growth mode until Wednesday at 7. p.m. After 48 hours, though, the biological stimulus for your body to build new muscle returns to normal. That means it's time for another workout.
Turns out, this 48-hour period is also similar to the duration your metabolism is elevated after a heavy lifting workout. As a result, total-body training is highly effective whether you're trying to build muscle or lose fat. In fact, I'm convinced it's the single best way to burn blubber. That's because the more muscle you work, the more calories you burn—both during and after your workout.
The other strategy works well is an upper-lower split. This is mainly used for adding muscle size and strength and for improving sports performance. In this method, you work your upper body and lower body on separate days. The reason: It allows you to train the muscle groups of both halves harder than you could in a total-body routine. However, it also means that you need to give your muscles a little extra time to fully recover. For instance, you might do a four-day-a-week plan in which you do a lower-body workout on Monday, an upper-body workout on Tuesday, and then rest for a day or two, before repeating (on Thursday and Friday perhaps). That'd give you two to three full days of rest between each type of workout. Or you could alternate between lower body and upper body workouts every other day, three days a week.
Keep in mind, there's no reason to use a split routine if you're gaining muscle and strength with total-body training. But if you reach a point when you can't fit all of the sets you want to do into a total-body workout, it's likely time to make the switch. Or you may simply want to experiment with different methods, to determine what works best for your muscles and for your lifestyle. You'll find there are plenty of workouts in this book to keep you busy.
How many exercises should I do per muscle group?