How long can a 35mm film be used / stored before it starts to lose its quality after expiration?

One of Photo.net's users is a chemical engineer who worked at Kodak for about 30 years. He posted the following information in a discussion thread regarding the life expectancy of a refrigerated film:

In all cases where we use a film beyond its expiry date, the
the only safe approach is to try a roll of EACH PRODUCT and evaluate it
before pulling the rest of this product. The stability of the film
products is different for different products.

That said, here are some general guidelines. L & # 39; expiry
date for many products is about 2 years after manufacture.
Refrigeration will preserve the film's chemical properties for 2
at 4 times longer than at room temperature. If you bought fresh film
and refrigerated, the chemical properties should last 4 to 8 years
instead of 2.

Freezing will preserve the chemical properties for about 8 to
16 times longer than at room temperature. Frozen film can be expected
maintain the chemical properties for 16 to 32 years.

If you do not have access to a salt mine, the background radiation can not be
stopped by any process that each of us can afford. Background radiation
causes fog and grain increases in the shadows. All movies are
sensitive to ROUGHLY background radiation in proportion to the speed of the film.
In other words, an 800-speed movie would be about 32 times more sensitive than a movie
25 speed film. It's very difficult since the current speed of Kodak 800
the film is about 1/4 as sensitive as the generation of 8 years ago. All
these discussions about keeping the film in a refrigerator or freezer should
only applies to low speed movies (200 or less). With high speed movies,
the background radiation will degrade the film regardless of the
storage temperature.

FWIW, I shot K-64 which was in my freezer for 20 years with
good results. I do not shoot 800-speed movie that is only 6 months old
until expiry.

In fact, there is a ton of information about it on Photo.net if you do a search.

There are also some questions here that might be helpful:

Kodak and Fujifilm always produce high quality, fresh, reliable and beautiful color films. Take a look at (for example) Freestyle photographic supplies to see other current manufacturers. I encourage all aspiring movie users to buy new movies – you do not have to worry about storage conditions, and you support the ongoing making of the film.