I thought after the first date it was a possibility, I knew for sure after 6 months.
The one thing I can say for sure though is don't rush it, If you think it may work then move in together and actually live together for a while, like someone else at the start said 5 years is a good amount of time, 2 -3 years min I would say before you truly commit to the marriage idea. That gives you lots of time to get to know one another as a couple, every one has their faults and pet peeves but many you don't know till you live together and many of the "cute" things she or he does can turn into really annoying things once you live together. If you can stay happy living together for 5 years you know it will work and it's what you want.
Thats the best advise I can give take your time before you pop the question and be sure it will work in the long run.
I met my soon to be wife at a friends place and we hit it off, we moved in together about a week after we met (very fast I know). We have been together now for 6 or 7 years, have one 3 year old son and another baby on the way due late Nov early Dec., and still aren't officially married, I proposed last x-mas and were now planing for next summer money permitting (man kids cost a lot) but at this point we know 100% it's what we want and that we can live together happily.