pulling the injectors every 500 hrs for a check (one good reason for this, as Paul mentions, is to prevent them corroding up in the bore). Running a good grade of injector cleaner every third tank or so, especially when the engine is used at low rpm for battery charging can be of great value. One of the major headaches for mechanics maintaining the various charter fleets is low rpm operation for prolonged periods of time. A good injector cleaner will help prevent this problem as far as the injectors go, but does not help much with carbon buildup in the exhaust system, which should be addressed at least every 1000 hrs. Many injectors will test "marginal" but run well for thousands of hours - test facilities will seldom tell you your injectors are perfect. A regular regimen of cleaner in the fuel can keep injectors in excellent condition for many thousands of hours. I currently have a cummins engine in my tow truck that has 287,000 miles on the original injectors ( I also run a bit of two cycle oil in the fuel as well to keep the injector pump well lubricated).