Unfortunately, you lose fitness far faster than you gain it. Fortunately, you have a grace period before your body stops maintaining your strength and turns you into a winded blob. This gives you roughly a week to ten days to resume your workout routine without serious fitness loss. Once you start up again, you'll feel sluggish at first, but your underlying oomph will be there. Just don't let your slack period go too long; After four weeks off, it can take up to 12 weeks of steady work to regain your form.