Q. How long can you keep a used roll of film in your cupboard before having it developed?
Cold and dry environment. Exposed photo films will immediatly start a chemical reaction catalized by light. I have been successful in conserving exposed films for around 6 months, no more. After that, the grain turns very strong and colors start to vanish.
Chemical photo films are a mixture of light sensible silver salts and colorants, on a layer of polyester and gelatin. Even unexposed, they react (very slowly) changing their composition and decreasing their light sensitivity. Industry has successfully achieved times of stability for periods of 5 to 10 years. The cold and dry environment found in a home refrigerator or freezer can enlarge these times BUT NOT to stop them. Once you buy the films and rush them into the refrigerator, the expiration date can be surpassed for more a couple of years. This is longer in controled refrigeration systems found in museums, for example, but with devices and equipments far unaffordable for a common user. Another issue is that once you choose this mean of preservation (at home) it MUST be kept in a air sealed container to avoid food contamination and vice versa. Before charge them into camera, a minimum 2 hours time period in room temperature MUST be observed in order to avoid water condensation in camera interior, which could impair the life of camera components. Nowadays, the purchase of great amounts of film is hardly economic viable. I suggest just buy the amounts you need for immediate job. Even in a "digital" age, the film prodution will naturally decrease but will probably not to discontinue for several years.
Disclaimer: These are my personal experience with films in the last 20 or so years BUT I cannot assume any responsability for misuse or hazardous issues come from home films conservation.