I currently work in a shop and teach fulltime, but I was very lucky in finding this position. My normal week is Mon, Wed, Thur, Fri then Mon, Wed, Thur, Fri, Sat. This is a realtively small shop with the only instructors being the owner and myself, we get along very well and it's not a dog eat dog world when it comes to getting students.
Pay: as was previsouly stated most is not very good, but one thing to keep in mind, like most sales positions there is commission involved. Another thing to do, if you are not already aware of this, is to upsale yourself.
Dropout: I am not sure where you came up with that. Most instructors that I know do it part-time. The few full-time instructors that I do know, have an unbreakable love for the sport. One thing I do suggest doing if you start to feel burned out, is to starting taking classes yourself where you are the student. Not insturctor classes, but diving classes, i.e. rebreather, tech, etc. So, you know what it's like being the student.