Does a longer immersion heater last longer?
The lower watt density on the longer element will reduce the thermal stress on the heater as long as it will fit. Having a lower watt density should equate to longer life with all other things being equal. Make sure your anode is in good shape when replacing the element for maximum life of the element and the tank. Added:Your heater life may be short because the anode has eroded away. Usually they are attached on the top of the tank a rod that is made out of metal designed to be consumed instead of the tank, heater, and plumbing being erroded. The anode should be checked every few years, I find they usually last ~3 years and many are gone in 5 years. Check the anode and probably replace much cheaper than a new heater, and you just might find your heat rods lasting twice as long or longer.