I agree that there isn't a set timeline for things like that. If you feel comfortable enough with someone and you don't think they will judge you, that's the time. Trust your gut! I hope you can feel good with someone enough to open up to them.

Personally, I think bipolar is one of those mental issues that people aren't too terrified of. It's easier to understand than others because (my understanding at least) the public has more knowledge about it than some other things. That's just my opinion though.