He and I have talked about our thoughts on the subject, but we've never seriously discussed it. I'm not sure that I ever want to get married, but in order for my significant other and I to be together long term, marriage is going to have to happen (different countries -- visas are needed for us to live together). We're not at that point, though, so we haven't spent much time worrying about that.
How long two people should be together before discussing marriage.... well, that all depends on the couple. There's no set time frame.