I... I can't remember, exactly.
Okay. I remember that I first thought about becoming a vegan, paradoxically, after reading a Matthew Yglesias post which was actually anti-vegetarian/animal rights, but he called vegetarianism the "desperately inconsistent sister" of veganism. (I'm not a super huge fan of Matthew Yglesias's rather indifferent views on animal welfare, but he seemed logical on this point.) I can't find that Matthew Yglesias post anymore -- he's moved servers a lot, with the typepad account, TPMCafe, etc.
Some time after reading that, I became a vegan while I was college, but thought that it would be too hard on my mom to be a vegan at home.
I actually started being a vegan at home... sometime last summer (i.e., summer '06). Not sure when, exactly. I was talking to a counselor about some problems I was having, and incidentally I mentioned to her that I thought that it was bad that I ate eggs and dairy, but I didn't want to make life hard on my mom. She said that I should be more assertive, and tell my mom that I'd be willing to cook for myself if need be, and it needn't be any of her business, but I wasn't going to eat eggs or dairy anymore.
So I suppose sometime in the summer of '06. So about nine months.