IME, until the sprocket bearing (in the tip) fails. With proper
sharpening and chain-lube.
Dull chain really shortens bar life; ditto lack of lube.
Cutters dulled on one side (e.g. from stone contact) can wear one side
of rail, making chain sit at an angle. Then you have a circle-cutter.
Pretty easy to check and true-up, so long as groove is deep enough to
let chain sit on rails after.