Injector hour life

Anybody have an idea on how many hours a set of injectors is good for in a 5.9 cummins? I know that is kind of a loaded question but just looking for a rule of thumb to go by. A friend of mine had the head off his D5C awhile back and thought he would have the injectors tested while he was waiting on some parts. They all tested bad on the spray test even though the dozer seemed to run fine. He ordered new ones and says he can't believe the difference in power. It just got me wondering.

Injector hour life