Until your salary gets too expensive!
This is true, if your school likes you... they can renew you until you die.
But let's not lie... they like money more than they like you, so when your salary gets too expensive they will want to bring in new blood... because it's cheaper!
At that point, you can just leave, or you can negotiate with them. A "salary freeze" or something. Take a small paycut, whatever it takes. If they like you, they will happily negotiate. It's usually about money.