I went out with someone who was BPD who didn't tell me for a good while. She waited until I pretty much found out myself but I never held it against her. I can empathize and it's hard to expect the majority of people you meet to stick around if you have to introduce yourself and your condition every time. I know you probably feel guilty that you are creating a connection that could falter but you have to realize that he has to take some of the responsibility for creating that bond. I was able to understand after she told me her guilt for "sucking me in" like this new guy should too: I chose to take her into my life because I cared about her.

So I don't think you should necessarily put a timeline on it. I do think you should eventually tell him if he's important to you as I'm sure he'll see some of the behavior himself anyway. Of course I was distraught when she told me but I realized that she was still the same person I met. I just began to notice her behavior patterns. Best of luck!