Humm, depends on the people. I've been going out with my gf for almost 8 months and we have no plans whatsoever to move in with each other we've never even talked about it...so..
My advice is to wait more then 3 months though, you'd be surprised what you don't know about a person. But if you guys are cool with it, then go for it. Just try not to get on each others nerves, it can happen pretty quick. lol