-- The first time I started riding in college, I think it was 6 months (2x lessons a week) after we first started riding. Honestly, we could have very easily started about 2-3 months after we started, but we were in the Walk-Trot IHSA class, and my trainer wanted to make sure we had VERY solid seats before he started us cantering.
-- Currently, it took me about 5-6 weeks (10 lessons) to go from not having been in a saddle for 10 years to cantering. Even then, that was with my trainer pushing me a little. I'm not afraid of cantering per se, but I'd like my balance to be better. (My trainer would probably add here that I'm crazy and my balance is fine.
I think it all depends on the student/trainer and the horses involved. I don't think I'd personally ever do that, but so long as she doesn't have you jumping 3' oxers during the next class, I don't see it being a problem with it. So long as *you* are comfortable with it, I think that's all that matters. If you feel balanced, if you can control your horse and trust him/her, I think you'll be fine.