I gamble alot on expired film.
B&W is quite robust and if it's a 100 speed film I will go out to about 10 years expired.
400 speed is a bit iffyer (a new word)
I usually will pass on high speed like Neo1600 or TMZ if it's outdated at all.
Now color materials are a different story.
For c41 I'm usually comfortable to about 5 years expired but have shot older.
E6 can be a crapshoot. If I'm reasonable sure it's been frozen I'll trust 5 years expired.
One odd thing is I've noticed some older style Kodak E6 like EPP seems to keep well.
I try to not pay more than 2USD for expired stock no matter what it is.