you can only bill 8 hours of flight time a day, 40 per week, so I would imagine for the very industious, they could BILL up to 12 to 14 hours a day. Then work 7 days a week, running ground schools when you are out of flight time.
Theoretically, that would give you 4,380 to 5,110 hours. Making say $35 an hour, you would make more than alot of doctors.