What type of relationship is it? Not that it matters that much anyway, just curious...

I would wait until you feel like you know the person really well, for one reason: if you tell someone you have a mental illness often they define you by it, rather than really getting to know you first. There's a difference between being bipolar and having bipolar, and it's helpful to let new people learn that difference on their own.

I used to tell lots of people, casually, but now I like to keep it more private. I don't think it makes me a bad/weak/etc person, but it's a tough thing for people to understand. Before I was diagnosed I found out my good friend in high school was bipolar, and it kind of freaked me out... but we were good friends by then, so it didn't cause me to withdraw from her at all.

Jess