There is no "right" time -- whenever you feel comfortable, I suppose. My current boyfriend and I had been friends for 2 months when we started dating, and I told him during week 1 or 2. He'd had a chance to see me as I was -- flawed but independent, able to take care of myself, etc -- without my diagnosis clouding his perception. At the same time, I told him early enough such that (i) I wasn't being dishonest by withholding information, and (ii) he could walk away if the diagnosis was too much to take.

In general, I think weeks 2-4 are a good time to tell someone, but that's just my personal opinion. I don't count by dates, but rather, the duration that someone has gotten a chance to know me.