I've known teachers who taught 3-4 courses at a time. So 3-4 sessions/wk (7.5-10hrs), then you could get a couple hours of tutoring (lets say 5 but thats probably reaching), proctoring is almost always saturdays so you couldn't do too many of those - I've managed 2 on a single saturday (one morning, one afternoon)- those are even paid not by hrs (so $45-100/proctoring session, they take 4-6hrs depending on the test). But proctoring is in seasons (SAT season is biggest).
So assuming your the best teacher they have and they give you high preference for classes, and you're free whenever they need you (be it morning class, night class, afternoon class, weekend class). During the heavy season of MCAT AND SAT prep you could probably pull in about 20hrs a week. I can't really imagine you pulling off much more than that unless you learned more than just to teach the MCAT or became a master teacher or something.
It really is a part-time job. I've never met someone using it as a full-time job. No matter how many classes they taught.
And remember its still seasonal. If you go in thinking it will be your only job you'll be sorely disappointed when there is no MCAT prep from Aug-Sept or so. No SATs from Mar-Sept. And not a lot of tutoring chances then either. Its a very part-time very seasonal dependent job - not safe for an only income.