On one level, I knew my husband was "the right one" right away. He and I had a mutual friend who picks her friends very carefully - that was a good character reference.
I think I realized he was "The One" within the first few weeks that we were dating. We just had so much in common (he called me a female version of himself!) that we both felt it was just meant to be. We didn't want to rush into anything, though - we'd both been burned before.
When he asked me to marry him, it wasn't a total surprise. We had been dating for almost 10 months at the time. We then proceeded to have a 2-1/2 year long engagement, but that's another story