The final component in the FITT principle of training is time - or how long you should be exercising for. Is longer better?
Cardio Respiratory Training
Individuals with lower fitness levels should aim to maintain their heart rate within the target heart rate zone for a minimum of 20-30 minutes. This can increase to as much as 45-60 minutes as fitness levels increase.
Beyond the 45-60 minute mark there are diminished returns. For all that extra effort, the associated benefits are minimal.
This also applies to many athletes. Beyond a certain point they run the risk of overtraining and injury. There are exceptions however - typically the ultra-long distance endurance athletes.
In terms of the duration of the program as a whole, research suggests a minimum of 6 weeks is required to see noticeable improvement and as much as a year or more before a peak in fitness is reached.
Resistance Training
The common consensus for the duration of resistance training session is no longer than 45-60 minutes. Again, intensity has a say and particularly grueling strength sessions may last as little as 20 - 30 minutes.
Perhaps the most important principle of training (that ironically doesn't have it's own letter in the FITT principle) is rest. Exercising too frequently and too intensely hinders the body's ability to recover and adapt. As a rule of thumb, the harder you train, the more recovery you should allow for. Unfortunately many athletes don't have that luxury!