Film, like any other organic material doesn't keep forever. Since the emulsion is beef gelatin based, you can imagine why.
It'll keep reasonable well in dry cool areas for quite a while, hovever after exposure you need to process as quickly as possible, for the simple reason that once exposed, the silver halide crystals start developing all by themselves, albeit slowly. What we do by processing the film is to speed up that development by a factor of many 1000's. It also adds a measure of predictability to the process.
If you take a photo and just leave it, after a few years, areas of the film will be 'developed' whereas other areas will not - it all depends on how much light the film was exposed to, temperature, humidity etc.
Ideally you should process within hours.
When I used film for studio work, all film was bought in batches. One roll was tested and subjected to sensitometric inspection and the various graphs plotted in a database. The film was then kept in a freezer below -14C. This was long-term storage. We then moved selected batches to a refrigerator, no more than 24 hours before its intended use. The film was then processed after the job, no more than 4 hours later. Eack set of negs was also subjected to sensitometric inspection and the resulting graphs checked against the master copies in the database.
This was we could see if there was any deterioration in film stock and act accordingly.
One of the reasons I love digital so much is that all that hard work (and it was bloody boring too, I can tell you) no longer needs to be done, as long as you follow an established workflow.
