Well that's a tricky one...I don't think you can put a set time on it, but IMO it should be before things start getting serious (so he doesn't feel like you've lied to him all that time), but after he's got to know you a bit so he's less likely to freak out (i.e. he knows that you're not an axe murderer just cos you've got a MI lol) Congratulations on the new relationship though!

The way I thought I would do it, is to bring up my interest in psychology & MI early on with my career. Anybody that was still stigmatising after I'd tried to explain common misconceptions (if necessary), would not 'make it to the next round' so to speak. Maybe something similar would work for you e.g. you could mention something MI-related in the news or something?

*Willow*