Ben Boudro, strength coach at Xceleration Sports (Auburn Hills, Michigan), was once told that he should train at least 2 1/2 hours every day if he wanted to get big. The longer the workouts, the greater the results.
In retrospect, Boudro says, "That was terrible advice. Personally, my workouts are 45-55 minutes max. I don't do a set and then sit around and talk about it. I have a set plan every time I enter the weight room. I get that plan done and I get out."
The general guideline is to strength train for between 60 and 90 minutes for sustainable results. If you work out for too long, your body may start using muscle instead of carbohydrate and fat as a source of energy. Also, the stress from excessively long training sessions is one of the primary causes of overtraining.