FWIW, I've got 251K miles on the (AFAIK; there's a 20K mile gap in the maintenance records) original injectors in my car, and everything's happy.

I think that injector longevity is a function of the quality on the injectors themselves, the quality (lubrication properties) of the fuel used, and the efficiency of the filtration system. "Indefinite" may be asking a lot, but some injectors do tend to last longer than others. As the posts on this thread show, M-B injectors typically have a long life-span. The Stanadyne injectors used on '83-'94 Ford trucks (and I believe similar injectors were used on GM trucks in roughly the same timeframe; I'm sure rwthomas will correct me if I'm wrong), however, have a much shorter lifespan (probably in the 100K to 150K mile range), and unlike the M-B injectors, are not feasibly rebuildable (at least not by a DIY; there are shops that rebuild and re-sell the injectors).

Just my $.02...whunter probably said it best, though

Good luck!