Train 3-4 days per week and directly stimulate each muscle group once per week. The best approach is to split your body up over the week rather than training your whole body each session. Here is a great training split that I highly recommend:

Day1: Legs/Abs
Day2: Rest
Day3: Chest/Shoulders/Triceps
Day4: Rest
Day5: Back/Biceps
Day6: Rest
Day7: Rest

This split allows plenty of rest in between workouts (remember, you grow OUT of the gym, not in it) and eliminates overlap.

I would suggest keeping each session to a maximum of 1 hour, and performing 5-7 sets for large muscle groups (chest, back, thighs) and 2-4 for smaller ones.