Depends on the film and storage conditions. The rule is of course to process ASAP after exposure, but most consumer films are good for weeks or months - remember, there are huge numbers of people who buy one film and make it last all year
Pro films (eg. Provia) are less stable and will deteriorate more rapidly, but no absolute figure can be provided as the environment is a huge factor.
I once came across 3 rolls of Ektachrome in the back of an old cupboard which I had shot 5 years earlier and somehow "lost". Since the films were shot in Berlin on the day of re-unification, I decided to get them processed just for souvenir value. The resulting slides all looked a little over-exposed and faded, but the colours hadn't noticeably shifted much and a little work in Photoshop after scanning produced some useable images.
Colin K. Work, Pixstel