| Tough to answer. I have had heaters not last very long, and I've got a Tronic 75w heater that I've used for about 12 years. Personally, I feel the temp gauge is probably the weakest link in any heater and the fist thing to fail, either by dying or becoming innacurate.
Using a heater controller will give you a lot of piece of mind. It will ensure the temp is maintained correctly, but will even allow a heater with a failed "on" temp controller to still be used. | 
