A glow plug is somewhat like an incandescent light bulb.....
throw a butt load of electrons at it .....and it gets hot - real hot!!!
I light bulb will almost last forever ....if never turned OFF
- the heating and cooling of the filament causes the "metal" to crystallize and thus the life span starts to reduce.
I would think that a glow plug would be much the same - the flow there is a lot more than the light bulb however!
Then again - a glow plug is almost identical to a heating element placed in the water to warm the engine. But the difference is pure resistance - a glow plug has damn near ZERO resistance, thus the current draw in tremendous! Where as a warming element has a higher resistance and is not getting red hot - but very warm, they are left on for hours at a time.
I think I would (after finding WHY is was on) have the dealer replace them just in case!