It depends on your level of fitness and how many exercises and sets you are going to perform.
For all body weight training per session: 50 – 60 minutes is a right time.
Don’t forget, that you have to:
- Warm up for at least 7 min before workout
- Stretch for 5 min after warm up
- Cool down for at least 2 minutes at the end of the workout
If you are pressed for time, you can do a primary weight training session at the gym, and finish at home (push ups, abdominal exercises).
The key is to perform quality session and to perform the exercises with serious effort.