Simon, the cylinders should last you ages, providing you get the piston ring gaps and piston to cylinder bore clearances right, and you warm the bike up correctly and run at right operating temp (no cold seizures) and you don't run lean.
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Rich
MC21 Track Bike / RS250 NF5 'Spencer', NX5 'Cadalora' & NXA 'Aoyama' / RS500 / Two Brothers Racing RC30