GOOD glow plugs can last a LONG time... if you properly tune the engines. I had one plug outlive 3 engines, from break-in to worn out completely. (unfortunately they don't make that plug any more. Or it would be all I ever bought.)

Never let the engine run lean and you shouldn't have problems if you are using a good quality glow plug. The O.S. plugs seem the best of what I can find now. (Up to 10 gallons through a .40 size engine per plug.)