Well, I can't consider my self a successful mathematician, but from observing a few ones I know, here is my 2 cents:

The 10,000 hour rule (this is of course pseudo-science) certainly apply almost by definition: most mathematician start training in college if not before. If you count up to postdoc, which is very typical, that is about 12 years, and an average of 3 hours/day give you 10,000 already.

Certainly work habits vary, but it does seem that quite a few successful mathematicians I have met know how to enjoy life. Having said that, I think a blessing/curse of our profession is that the lab is in our mind. So it looks like we do not work that hard, compared to some other fields, since some of my friends in physics/biology have to stay at the lab at nights frequently because of experiments. On the other hand, math can follow you around even when you are playing tennis (I can confirm that from personal experience!).