Lifetime of the tensioner is determined by age, heat, and environment (high ozone), since the active part is rubber. Mileage plays little part. Tensioners that rely on metal springs would probably last longer, with the bearing being the only real weak point.

Steve

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'91 MB 190E 2.3
'08 RAV4 Ltd 3.5
'83 Lazy Daze m'home 5.7