I assume you're writing this for new members, because I've answered this many times in the past. The answer to 'when' varies. Sometimes it can be a week, sometimes a few months, sometimes never if it becomes clear that they are firmly against it. I wait until I know how they feel about the subject. Over time, I will gradually bring up topics about known alternate lifestyles and the celebrities that live them. Ask if they have anyone in their family who is lgbt. Ask how they feel about them; are they close, are they disgusted with the idea, how they feel about knowing someone who's sexual life is so different from her own. While watching TV tune in so that maybe Rupaul's show (or an episode of some talk show with a lgbt guest, family or child) comes on next, and discuss the people on it with her. Caitlyn was an excellent topic, and I've used her as a reference with several women to see how they feel about this. Continue until you hit the point where she says something to the effect that she could never date/marry a man who behaves that way (or if very, very lucky, and win the crossdresser lottery, find out that she's open to sexual diversity in her own life). Then we have the option of outing ourselves to her (either if we think she may change her mind which isn't usually likely, or just as a way to end it), or just finding a way to end the relationship with some other reason, and it can't be something that denigrates her (my most common excuse is that I was contacted by my ex, and want so see if we can patch things up; I think most women can understand that we still care about our exes at least a little bit).
Initially, it was difficult for me to do this. it took a lot of planning and coming up with scenarios in order to 'feel out' how the woman feels about this. But over time, I've gotten good at it. I'm not doing this to hurt anyone. But I don't deserve to be hurt, myself, either, just because of other people's ignorance. That I refuse to tolerate. I've been hurt a lot in my life because of that, and I won't put up with it anymore. If someone thinks I'm less of a deserving human being just based on gender dysphoria, and don't believe everything they can observe about me and what I tell them, then they don't deserve my respect, and they should be happy that I'm so careful to do my best not to hurt their feelings. It would be much easier to just stop calling them or come up with something hurtful that they can't change as the reason for ending it. But I don't do that.
I get the feeling that either you forgot why I didn't tell my wife, or again, just wanted it to be told again for newer members to read. So again, in short, I believed that I had 'beaten it'. I hadn't crossdressed for several years when I met her, and didn't for years into the marriage. Like many other things, it was in the past, I had hurt no one, so I didn't think it was something that she either needed to know, or would want to know. Much the same as she didn't tell me that she was treated for major depression before we met, that she has passive aggressive tendencies, or that her mom was an alcoholic. What I find fascinating is when women defend her 'omissions' justified, but mine, not so much.