When should I tell someone I'm dating that I have bipolar disorderr?

No one right answer. This is a tough question. It can be a turn-off if it comes up on the first date, but waiting till right before wedding planning is also not a good idea. Bringing it up when you feel you and your partner are getting serious about your relationship is probably the best.
When it's serious. When your relationship is, or looks to become, serious is a good time to discuss potential obstacles of many sorts. One possible way of answering this question for yourself is by asking yourself: "when would i like to know", if the situation were reversed?