Muscles begin to get smaller and weaker within a short period of inactivity. Astronauts in space, where there is no gravity and little stress on the body, lose muscle tone in a matter of days.
Bedridden people also lose muscle tone, strength and size. People who work desk jobs and don't exercise are at risk for muscle wasting as well. The good news is that muscles can be built up after periods of inactivity.
Weightlifters who stop lifting for a while but continue with normal daily activities don't immediately lose their strength. It may even be good for their bodies to take occasional breaks from weightlifting for one or two weeks.