Probably since the late 1990s, which means before something like 90% of Democrats who support it now.
shua, I understand what you're going for here, but I think I can speak for all gays who aren't parody troll accounts here by saying that we recognize that people have to evolve on this issue, just as Obama and our family members have. While Hillary Clinton was unusually risk-averse, she's come around and become a strong advocate, and that's what matters now.
I voted for Hillary over Obama in 2008 in part because of Obama's reluctance on this issue—at least with Hillary, I knew I would get caution and cynicism. Obama has surprised me by embracing gay rights in his second term.
That is the case for most gays that I know personally, but I keep hearing from some corners that all those who oppose gay marriage are bigots. And that if you don't believe that those who oppose gay marriage are bigots who deserve to be punished, then you are a closet bigot yourself too. That is not what I would expect from those who recognize that this is a complicated issue for people they need to evolve on before they can embrace it.