There is one other possibility but with todays glow drivers, remote. If you have too high a voltage the plug life will go to poop. It should glow a nice cherry red to orange, If it's white it's too hot.

Look very closely at the exhaust residue. If there are little sparklies in it that is an indication that something is wearing inside the engine. Those metal particles are murder on a glow plug.

However 99% of short lived plugs is too lean, or to much nitro.