The NASM recommends no more than 90 minutes per day. That totally depends on what you're doing and where you're at.
Overall, key factors are stage of fitness, fatigue, time and enjoyment.
Beginners should start with less time and less difficulty, obviously. As we progress, the body adapts and those shorter, easier work outs will do almost nothing to progress you to the next level so you must increase intensity, difficulty and duration.
A person who has been active for years needs to concentrate on mixing up their work outs in monthly intervals. One month, they may be working on stablization and endurance. Another month, hypertrophy. Another month, power and agility, making sure biweekly to work on all the different areas.
The most important thing is to mix it up so that you like what you're doing, or at least don't hate it enough to skip working out altogether. And to keep it at a level that is safe for you.
To sum it up, if you have 30 minutes every day, fine. Vary your activity enough to avoid over injury. If you have 90 minutes 2-3 times a week, do total body work outs in a circuit to get cardio and resistance training. Do what works for your schedule and for your mind.
Last edited by fitness4life; 02-05-2012 at 05:03 PM.