For every day off, for example, what percentage of your overall fitness level decreases?
It's going to vary from individual to individual. Most people can take 7-10 days off with no appreciable drop in fitness. After that, the general rule of thumb that I learned in college and has been borne out in experience, is that you lose 2 for 1 after that.
So, if you are out three days past whatever your particular threshold is, it will take you 6 to get back to your original starting point.
Also, (And again, this is personally dependent), if you are out for 30+ days, you are generally considered to be detrained, and starting over somewhat from scratch. You will progress more rapidly back to fitness than someone that hasn't been fit, however, and the length of time that you originally did whatever sport you are in also reduces that. So if you have been a runner for 15 years, and take a few months off, you will get back to your baseline much faster than someone that has never been a runner.
Age also plays a factor, in that as you get older, it takes a bit longer to get back to where you were, and depending on length of time out, how old you are, injuries, etc., you may not get back completely to your original starting point.
I think there is a question on the site that addresses it both from a fitness and a neuromuscular standpoint, if I can find it I will link it.
Not completely the one I was looking for, but: For how long can I stop doing cardio training before losing my endurance?