The key component of this is to take one--an off-season that is. All athletes, especially those that work extremely hard for most of the year need some time to recover, both mentally and physically. As hard as this is to the 'hard-core' athlete, it will improve performance down the road and increase your athletic longevity.
The time to do this is normally after the final big race of the year. When we race, the stress caused on our bodies is such that it often creates micro-tears, slight muscle strains and other physiological ailments. To train through these can cause these problems to worsen and eventually become major debilitating injuries. Often, these problems develop somewhere within the last quarter of the season when the training and competition is the fiercest. Since most training schedules do not give the body the kind of time necessary to repair these injuries, some time off is needed. How much is needed depends on a slew of variables like how hard of a training cycle you were on, how much pain you are feeling, your age and overall health, your body type, the event(s) you competed in, etc.
An additional reason that some rest is needed is based on an athlete's mental state. Many athletes become 'burnt out' psychologically at the end of their season, and need to give their minds a rest in order to get pumped up for the next cycle. To skip these breaks can often be a detriment to the next season's training regimen and can also lead to a complete 'burn out', where the athlete cannot continue to get motivated to train hard and compete effectively. Even if you think you are not burnt out, it is a good idea to give yourself a little break (We have seen it happen to way too many athletes for it not to be a factor!). If anything, the anticipation of getting back into it will make you that much more psyched up when you start up again.
Now, for many athletes, not doing anything is just not an option. They go bonkers and feel like slugs! To them, we recommend cross-training during the off season. Athletes doing this should try to do exercises as dissimilar as possible to their normal sports in order to give the key muscle groups a rest while not sacrificing cardio and mental benefits.
Finally, the amount of rest you decide to take is up to you and your coach or trainer. Depending on the physiological variables listed above and your mental state, you may want or need only a couple weeks rest--which we feel is a minimum. For others, a rest of over a month is acceptable. On the other side, don't be too much of a slug and heathen during your rest. 15 pounds gained on the off-season will hurt coming off when you start up again, and can lead to injury. Let your body and mind and coach collaborate on this decision and you'll be happy you did!