When does REST become DETRAINING?
There have been a few studies done on detraining. How quickly you lose fitness depends on how fit you are, how long you have been training, and how long you stop.

From the Mailbag:
I imagine it will be very different for different physiques, different metabolisms, and different people. But, is there a GENERAL formula that we could subscribe to? I ask because I travel frequently for work. While I always attempt to fit in workouts of one sort or another, it’s not always possible. So, I try to plan my rest days and or easy weeks around this fact. But, I'd like to know where rest is no longer helpful, but actually becomes detrimental (i.e. detraining).

Answer:
Hi and thanks for the question. You are 100% correct that athletes will all differ on how much fitness they lose. How quickly you lose fitness depends on how fit you are, how long you have been training, and how long you stop.

Detraining studies
There have been a few studies done on detraining. One group were exercisers that have been training for one year and then were told to stop training for three months. These athletes lost about 50% of their fitness. People who were newer to exercise had it much worse off. In this study the new exercisers were started on an exercise program for two months and then told to stop exercising for two months. These people made huge aerobic gains during the two months of training, but when they were re-tested after the two months off, it was apparent that they lost almost all of their fitness gains! More...