I think 3 years is good, but it does vary by couple. I think it's a good idea to live together first though. Sounds stupid, but that's when you really see who they are! For instance, I found out how terribly unorganized my partner is when we moved in together. It hasn't changed after almost 2 years of living together, and it will never change. I've chosen to accept it, but it would have sucked to marry him, then find that out, and be unable to handle it.