The life of the elements depends on

1) Fluctuation in supply voltage:  For every 1% increase in voltage, there will be  a 2% increase in power.

2) Watt density of the heating elements.

3) If more power is needed, increase the diameter of the element wire or reduce the length of the element.

4) The elements must be operated at the design voltage.

5) For the cyclic nature applications elements need adequate space to move on their hangers or supports.     Elements should not be located close to the bottom or to a refractory shelf because expansion may     cause them to rest on the refractory where heat will not dissipate from the element, allowing a hot spot     to develop.

References:  

O.P. Gupta: elements of fuels. furnaces and refractories.
 Daniel H. Herring   dherring@heat-treat-doctor.com