A lot of people think that really long workouts (2-3 hours) produce more results than shorter workouts (45 min-60 min).  Research has shown that after about an hour of working out, your testosterone levels drop.  Testosterone is a hormone produced by the body that causes muscle gain and fat loss.  During weight-training your body produces another hormone called cortisol, which can actually destroy muscle and cause your body to store fat.  After an hour of working out, your cortisol levels rise and your testosterone levels drop.  At this point, your body is actually storing fat and burning muscle for energy.  If you limit your workouts to about an hour, your body is much more likely to burn fat and build muscle, which is what most people want when they work out.

You might think that it’s impossible to complete your entire workout in an hour.  The key is to work out harder, not longer.  Try shortening your rest periods in between sets.  Instead of resting three minutes between sets, try resting for just one minute. Another way to save time is to work two different muscle groups back to back without resting in between sets.  Try doing a set of pullups, then immediately doing a set of squats.  Your upper body rests while you do the squats. And, in the process, you're shortening your workout time.  For cardio, try running or biking for 20 minutes at a fast pace rather than 40 minutes at a slower pace.

To increase results, decrease the time you spend in the gym.