Exposed film’s latent image starts changing almost immediately, but the effect is usually tiny at first. The biggest shift tends to happen in roughly the first 72 hours, after which deterioration is much slower.

How long it remains usable depends heavily on the specific emulsion and how it’s stored. Some films hold latent images very well for months with little noticeable loss, while others can fade much sooner. In the examples given, some modern black-and-white films such as TXP or HP5+ may tolerate months before processing, while films like Pan F+ can produce weaker negatives if left that long.

Storage matters a lot: cool or frozen storage greatly extends life, and exposed test films have reportedly remained usable for years when frozen. There are also historical cases of very old film still yielding images, though often with reduced quality and requiring careful processing.

So the practical answer is: develop as soon as you can, especially within a few days for best consistency, but don’t assume delayed film is ruined. The exact tolerance varies by film type, and cooler storage helps.