Friel mentions five terms used to in regards to training stress. You can find more info in his book Mountain Bikers Training Bible (see link in right sidebar).
This is how many training sessions are done. Beginners may train five to six times a week and experience 10 to 20 percent improvements. Sport to Expert (to pro) train as often as 2 workouts a day at certain times of the year. Seven to 12 sessions in a week are more common. However, these high frequency sessions may only produce a 1 percent gain in fitness, since these athletes are already so close to their potential. If a beginner tries to train at the the same frequency level, they would be overtraining, thus see a decrease in fitness.
Frequency at which you work out is dependent on what you body is currently adapted to.
This refers to the length of every training session. Anaerobic endurance intervals could last several hours, while recovery rides could be much shorter. Much like frequency, the riders experience will determine the length of the workout.
The appropriate time for long rides is largely determined by they anticipated duration of your races. Workout duration equal to, or up to twice as long as, your longest race are common. But there are obvious exceptions, especially at the high end. If you are training for a 100 mile trail race, your longest rides will seldom, if ever, be as long as the anticipated race.
Note: In mountain biking, duration is measured in time while road cycling duration is measured in miles.
Combining both frequency and duration refers to the volume of training, thus much easier to quantify. Volume of training is an incomplete description of a training session since it does not account for intensity. When you combine volume and intensity, that is referred to as ‘workload’.
Quantifying intensity is much more difficulty than quantifying frequency and duration. One way to quantify intensity is by use an RPE scale. The REP scale (ratings of perceived exertion) use a 1-10 measurement, with a 10 being the hardest level of intensity (race level effort). You can than multiple the the number of minutes by RPE and get a workload number.
Intensity is the stressor that most athletes get wrong. For most mountain bikers, getting intensity right is the key to moving up to the next level.
VOLUME VS. INTENSITY
Which is more important? Should a rider get in as many minutes as possible or ride less time at higher intensity?
The answer depends on level of experience. A new riders would benefit from higher volume while a more experienced rider should focus on intensity.