I have over 400’ of processed Kodachrome II ciné film (called “Regular 8mm” or “Double 8mm”)* that I made as a pre-teen kid in 1964 - from what I can tell, they look as good as they did back then. I keep them on reels in the dark, room temperature, but edited-out splices I keep in a box and they look good, too. So that’s 58 years.
* This is ordinary double sprocket 16mm film which goes through the camera twice, where each run exposes just half the width of the film (run it through once, flip it over, run it through again). After Kodak processed the film, they’d slice it lengthwise and join the two pieces into a single 8mm wide length of film.