The slower the film, the better it keeps. I try not to keep 400 or faster films around too long, but I've used slower films that have been more than 5 years past expiration without serious problems.
Occasionally I'll pick up really old film and try it, just to see what it's like and get a feel for an older film, even if it's considerably deteriorated. I had some Royal Pan expired in 1965, for instance, recently. It was a high speed film, so the base fog very high and tonal range fairly low, but the grain structure was really nice--almost a chisled kind of look. I also have some Double-X 35mm movie stock from the 1970s (I think) that I use occasionally. Base fog is high on that too, and speed is fairly low, but it's another interesting looking film, and the contrast is actually not bad, since the film has an inherently high Dmax, so there's still lots of room over the base+fog.