B1, who I met through OKCupid, asked me what I was after from the relationship on our second date. I said something along the lines of "what I'm not after is a happily-ever-after, never-look-at-another-man-ever-again thing" (I didn't realise there was a word for it at the time). As it turned out that was just fine with him. I'd read up on polyamory by the time I got together with B2 a couple of months later, and the fact that he's married kind of made it obvious from the start.
I don't think it would have been fair to wait any longer than I did with B1, but in my defence I was flying blind at the time.