Don't wait terribly long on that. Obviously leading things off with "I'm mentally ill" doesn't seem like a great idea most of the time. But once you've gotten to know each other reasonably well, you do want to be open with him. Otherwise it may seem like you were trying to keep things from him, especially if he found out some other way than you telling him. If he has a genuine desire to be with you as an individual, it'll be ok.