Like AKLIM said, the life of a GP depends on how many starts you make on it, how many cold starts and how cold it is where you live, but I would say under "normal" circumstances you'd be looking at 4-5 years or about 50K miles between changes on these cars as a minimum.

I changed all of mine in 01/05 when I bought mine and that was almost 35K miles ago and no troubles yet. I suspect they were the originals from 1998 with 115K miles on them but mine was a southern car and the GPs don't get as much time on in warmer weather as they do in colder climates so they tend to last longer down there.

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Marty D.

2013 C300 4Matic
1984 BMW 733i
2013 Lincoln MKz