Replace them when they fail.
I did a good 25,000 miles on the original, wafer-thin front tube, and over 30,000 on the rear.
The front tube is still fine, and I carry it as my spare tube. The only reason I swapped it was because my spare happened to be a thicker tube, and thus took up annoyingly more space to carry.
I changed the rear tube because I pinched it fitting a tyre. Otherwise it was still fine.
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I spent a lot of money on travel and racing motorcycles - the rest I just squandered.
XT660Z -
Commuter, Adventurer...Racer?
www.dashmoto.net
Other bikes >> MT-03 / CG125BR-J / ER-6F
Departed >> ZXR400L3 / EC300 / DR-Z400E / DR800S / GPZ500S / GS400
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