How long will exposed film last if it's not developed right away?
You may have heard of negatives from the beginning of 20th century, processed just recently. Much depends on the type emulsion and storage conditions. I never tried it with colour negatives, but processed some b/w German and US films from the times of WWII. The storage conditions were not so good, so I started cutting 5mm strips from the loading end until I reached the border of the first frame, than cut the rest onto separate frames, and processed each of them separately, soaking them in a pinakryptol green bath prior to developer, DBI (development by inspection), having a green filter over a 15W bulb.
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The latent image on film begins to deteriorate in minutes after exposure. We are talking minuscule changes of no consequence. The greatest effect of latent image shifting occurs in the fist 72 hours. After that, the shift is quite slow. Undeveloped film was found in abandoned encampment of a failed artic excursion. The film, nearly 100 years old, developed OK. I was once manager of a team that made test films for the photofinishing industry. We exposed under laboratory conditions. We exposed, aged the film 72 hours and then froze the un-exposed film. Clients developed the film using their routine procedures and the film was then compared to pre-developed reference film. Such test films will keep for years if frozen.
One of the differences between film designated as "professional" and that designated as amateur is, the amateur film is more robust as to latent image keeping. Not unusual in the photofinishing business to get film from two Christmases.
This depends on the film. Some films will retain latent images better than others. As best I can tell it depends much more on the particular emulsion than on things like the ISO: for instance TXP (ISO 400) retains its latent image very much better than Pan F+ (ISO 50): I often leave rolls of TXP or HP5+ months before processing with no real noticeable degradation, but if you leave Pan F+ that long then you'll get a pretty pale neg.
Note that all of this is for modern emulsions.