With my current boyfriend I never brought it up and after a couple of months at a party he started introducing me as his girlfriend, so I figured that settled it. A couple of weeks later he made it official on facebook, because everyone knows it isn't official until it's on facebook.
That's not how I've done it in the past, but I have to say it made things a lot easier. All I asked him in the beginning was "are we exclusive" and of course he said yes and I explained I just needed to be on the same page there to make sure if he wanted to see other people I wouldn't be surprised. In the past I've gone the route of waiting until I thought it was pretty obvious we were dating and then would straight up ask "what are we?" and discussed it that way, but even though I'd always act like I wasn't going to be bothered no matter what the answer was it was always never wracking to ask and to figure out when the timing was right. I think I'm sticking to the new method of waiting, though I do recommend asking about whether you're exclusive or not after a couple of weeks to avoid any unpleasant misunderstandings.