Well, I figured I'd post simply as the view of the partner who's being told:
It was... about 13 months into our relationship. Which, if I had been older then I was at the time, would've made me kind of angry that he waited so long. But we had taken things very slow. We didn't begin fooling around until month 8, and didn't have sex until month 14 (the month after he told me). So I was okay with the fact he waited.
But, if you do plan on telling your partner at some point in the relationship... I do suggest doing it in the first 6 months. No matter now hard that may be, an honest relationship is the key to any healthy relationship.
When my partner first told me that he wanted me to wear a diaper. I was- for lack of a better word- horrified. I was babysitting one of my neighbours children at the time and he had come over to keep me company. I asked all kinds of questions at first, and we basically, didn't talk about it for the few weeks following. Occasionally he would work up the courage to ask me to wear one, and I would just shoot himk a death glare and say. "No". He would sort of... shrink a little inside. I of course, didn't understand. And really didn't know it was hurting him.
We finally did decide we really needed to talk about it though, having shed a little understanding on it, I became more and more comfortable with it. Eventually, comfortable enough to wear one for him on special occasions.
Occasionally, I still get that, "This feels weird" feeling, when I wear one for him. So I've joined this forum to try and get even more comfortable with it.
If your girlfriend loves you, she'll accept you for who you are. It doesn't mean she'll wear one, but it means she'll love you regardless.
My best advice, is tell her. And make sure you talk about it so she can try and understand where it comes from.