There's no right or wrong answer to these kinds of questions--every relationship is different.
Living together is a bigger step for some couples than others. It's a level of intimacy that you may or may not be prepared for. If you're already in a very close relationship, physically and emotionally, it might not be so much of an adjustment. If you're already sleeping at each other's places more often than not, it's really just semantics. Then again...sharing toothpaste, toilets and household expenses is a lot less sexy than you might be envisioning. And it's an adjustment.
My sister and her husband (then boyfriend) moved in together after only a month or two...but the funny part was that they never really discussed it or officially made the decision. He just started staying over a lot. Even after she'd purchased a new DRESSER to hold all the things he kept at her apartment, it didn't dawn on her that they were living together until the day she saw him walking up the driveway carrying a coffee pot. It was just that natural for them.