Honestly...prior to installing all new Beru's I had a very long, good experiance with Champions >well over 10 years and more like 15 or so. Of course one would burn out here and there, it's to be expected.
I actually would 'carefully' hand test them using a battery...count to 10 and see which GP's came to life the fastest. Crude I know but it seemed to work at the time. Now I just replace when dead.
With the above being said...I just lost a Beru, 1 out of 8 that...I'm guessing this is either the 3rd or 4th winter coming up on them.
I do recall, my "original motorcraft/Beru's" GP when I bought my truck new in 1988 weren't in the engine all that long >and I replaced them w/the champions.
I think different elements must be taken into consideration "how long" a glow plug will last.
ie >condition of GP system, connections, how many times do you start your truck in a day and the list can go on!
So who knows how long they will last