It varies on many many things. Such as how fit you were in the 1st place, how long you have been that fit, how you managed to get that fit etc etc.
If you are in tip top race shape, a few days will be enough to knock your top end, but if you are moderately fit, have been for some time etc, then you generally won't loose much in a week. Following the PMC concept with default values, you would loose 1/42 worth of your Chronic Training load which is indicative of fitness per day (it is a concept, a tool, something you use and adapt to yourself, not something to be taken too literately).
6 days off knocked about 20W off of my 10 mile time trial power and I did a poor time (by my standards), however a week off and I can and did simply get back on the bike and casually ride 100+ miles.