My parents met when my mom was 15 and my dad was 17. They dated the whole time my dad was in college (my dad in upstate NY, my mom still in MD) and then got married when he graduated at 22. So he proposed after 4 years of dating. They've been together 32 years now.
I think 2 years is a good minimum to be dating before a proposal. I want to be married by the time I'm 23 or 24, so maybe I should start looking..