All of our CFI's here at Spartan get right around $13 an hour (I'm pretty sure the senior CFI's like Stage Check Airmen etc. get paid a nice chunk more but pay isn't something openly discussed out here). The only stipulation on the $13 an hour is that it is only time that you are with a student. If you have a 0700 flight that finishes at 0900 and the only other flight you have that day is at 1500-1700 then you get four hours of pay and depending on where you live you -might- be at the school that entire day. It -can- happen but we almost always coordinate everyone's schedules so nobody is left sitting around doing nothing.
All told you have the option of putting in up to fourteen or fifteen hours per day, seven days a week if you want to, the schedules are pretty flexible and nobody will expect you to do that but you can if you want.