Part of "getting in shape" is building muscle and the energy systems needed to feed that muscle oxygen. Most training programs are designed to get your physiology (musculature, vasculature, neurological connections) to adapt to greater physical demands. (Constant physical stress is not good though... training programs need to include rest periods to allow your body to adjust).
In general you'll start losing muscle after 10-14 days of inactivity. Basically if you aren't putting demands on your body to build or maintain muscle tissue it is too costly (in terms of energy and nutrients) to keep the muscle fibers around so they begin to atrophy.
I suspect there is a bit of variation in how long it takes individuals to lose their fitness and also a fair bit of variation in how long it takes to get back to a certain level of fitness after a long period of inactivity.
-traveller
"It never gets easier, you just go faster." -- Greg LeMond