Had Don service mine last year but i only have a couple hours run time on it (working too much). But if i was able to fish like i use to i would pull them every two years. It creeps up on you slowly so you may not be able to tell there is a problem till it's too late.
Couple hundred now or 20,000 later.

It's not just a dirty injector that will get you, the electrical parts will go out. Don found one of mine that would go lean and sometimes just stop working when they got up to operating temp's.