A couple of general rules:

#1: Aside from needed rest and recovery days (which increase fitness), additional days off will “cost” you 2.5 times the duration. So if you’re fully recovered and take additional time off, each day off will take 2.5 days to rebuild.

“Whenever the athlete stops training, a “detraining” process sets in, inducing a relatively rapid loss of the acquired adaptation. For example, half the increase in mitochondria from five weeks of training may be lost in one week of detraining. Since the number of mitochondria substantially determines the endurance capacity of the [athlete], it is obvious that the loss of one week of training will have a marked effect on the endurance.”
~ Jan Olbrecht, The Science of Winning, p. 8

#2: For every unplanned day off, plan on at least one training day below aerobic threshold. You need to rebuild aerobic capacity before adding anything on top of it.

I hope that helps.