A quality belt (Italian made Pirelli, German made Continental or Original Fiat) that is still in a box, hasn't been exposed to UV or sunlight, and hasn't been stored bent or kinked... I would say easily 20 years.
I know my Dad hasn't imported any parts from overseas for at least that long, and he still regularly uses his stock of belts, on some very exotic stuff too (Ferrari and Lambo) and happily uses the same parts on his own exotics.
I know when you open the box you still get a nice 'fresh' rubber smell
Not in a box and unknown history... I wouldn't chance it for the 30/40$ that a good quality belt costs compared to the cost of a top overhaul with bent valves... especially if it looks dry or dusty.