Well, I have actually never replaced a heater and that is 5+ years. I will be replacing all of them once I set my tanks back up (moved) since they were in a storage unit with rather extreme temperatures. But I also plan to use a temperature controller to make sure a broken heater doesn't boil my fish.
Once I do set my tank back up I probably won't replace until it breaks, since I will be less worried due to the controller.
many of the new heaters on the market are junk.