They last a while, depends on the conditions, how many times you insert/remove them and more importantly, how you store them.

Keep them dry and clean (no water - ever!) and you'll get a weeks commuting out of them. A hot,. dusty, sweaty cross country ride will give you maybe 2 or three days... For weekend warriors, the'y might last a month or two.

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Never underestimate the power of stupid people in large groups.