That depends on the brand of decals. I've got usable decals that are well over thirty years old. Tamiya decals, old school Monogram decals, and old school Matchbox decals have very thick film and still are usable.
Some aftermarket ones are more fragile because they have very thin carrier film. If stored properly, they can last for years and years. And if they look dried out, spraying a coat of clear coat will make them usable.
I have a loose leaf 3 ring binder with clear page protectors. The binder has dividers so I can catagorize decals by type; i.e. 1/72 scale armor, 1/35 scale armor, aircraft, sci-fi, etc. I put loose leftover decals in ziplock bags separating them by type, like WW2 German markings, white US stars, etc.
So if I go to my 1/35 scale armor tab, inside the page protector is several ziplock bags with Soviet, US, German and another with miscellaneous (like Japanese, British, Italian, etc. markings)
I also sometimes put the kit decals into small ziplock bags when they are loose in the kit box and not sealed. Helps keep them fresh.