It depends really... If you guys are still young, then it would be reasonable for you to wait until you are in your early twenties at least. Generally though, I would say two years in an adult relationship is expected, one where you are both still in love.
It really depends on what his views are on marriage, age, and how your personal relationship is doing. Keep in mind that marriage [should be] forever. If he doesn't want to spend forever with you then find someone who will. ~Ninja Jackie-san
How long have you been together? How long have you been waiting? Maybe he's just not ready yet. I'd either hint around with it, or just go ahead and ask him if he'd marry you :) Good luck, hope I've helped.
It does depend on how old and how long I guys been together. And then it really doesn't... Sooooo if ur really ready for that step here's what u should do, propose to him urself... Honestly men are scared to take that step, they second guess.... So as a strong headed woman I think u should propose to him.... If he says no that means he ain't the one for u and may be sad but don't let it dent ur ego. Think about it girl.
If you are truely in love, you may want to get married but if he doesn't ask you that shouldn't stop you from being with him. If you want to get married then talk about it. Drop some clues. Or ask him your self. If you are up to taking on that role.