Change them when they are sufficiently worn to hurt handling or performance.
For more important races, change them before the start if they might go off before the end. How long they can be expected to last depends on several things, you get used to roughly how long they last on your cars.
If they get chewed up (e.g. in a crash) so they cannot be straightened properly, replace them either immediately or before the next race.
If they get chewed up in a race, decide how much you loose by changing them straight away against how much you loose by pushing them straightish and continuing and do whatever you think will give the best result.
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