Well, since you gave platforms as an example, I'd say about 1-2 hours.
Most old platform games are this big, I think..
Unless it's a metroidvania, then with all the back-tracking it would probably double or triple the total length..
Anyway, if you're just making a normal platformer, don't think about the total length, think about how many levels it will have. 10 levels that you can beat in a hour is a lot more satisfying than 3 huge levels that look the same all the way..