I buy the cheapest Pearl Izumi bibs I can get, the Quest Bib Shorts. I had only two pair for about 18 months, and both are quite worn on the fronts of the thighs (sun damage?) but the chamois and the rest of the shorts are in great shape. I'd say ~14,000 miles split between the two.
I have 4 pairs now, and because I'm cheap to the core, If I'm going out for like 20-25 miles, I'll still wear the old ones. They ain't dead until they're dead.
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