In general...if it's past warranty (which is usually 1 year - replace). If something about a heater makes you ask this question...replace. It's 20-30 bucks - not worth the worry or gamble. Even if it was high $ heater, I'd still replace if something about it made me ask this question.
However, I think I'm going to trust my Eheims to go 3 years. But then again, I don't rely on the internal thermostat or cycle it often since it's only a backup if the controller fails on (and I also have a fail safe Auto Aqua Temp security if that fails). Even then I check internal thermostats at least annually (and if fail - gone). At three years though, even if passes checks I'll replace. I guess I look at the 3 year warranty as a 'heater coil/system integrity' warranty and really have no expectations that I can rely on the internal thermostat so designed system that way.