I agree with telling between dates 3 and 5. My ex was BPD but didn't tell me until well into the relationship and I felt extremely betrayed. I knew something was going on from her actions and the way she treated me, but she denied she had any mental health issues for over a year. It led to our break-up. My biological mom had BDP and was extremely abusive and it defined by childhood. Dating someone with the same condition as my mom just isn't something I can handle. However, there are other conditions I have no problem dealing with in a partner. Everyone has their own history and has different things they can handle (and not handle). I think a potential partner has the right to know that before getting into a committed relationship.