Since I'm married, I always tell people before the first date. But that's obviously a different situation.
When I wasn't married, I always got it out there pretty quickly, usually at or before the first date. My philosophy was that I didn't want to waste time (mine or theirs) with people who would never be open to the idea at all. I also always claimed that I wasn't looking for anything serious, just friendship and see if it leads to more. I honestly think that's one of the reasons I was able to find the love of my life, because I had finally stopped looking
But either way, I've found that people with no poly-experience are much more open to the notion of being friends and seeing if it leads somewhere... after all, there's no "monogamous" expectation of friends. Can you imagine??