XC racing - how often replace chain rings?

how often would you replace the chain rings on an XC race bike? My kids been racing and training hard for about a year on the same chain rings - just replaced chain and cassette, should I change the chain rings too? Looking at them its hard to tell as even the new chain rings seem to have different siz/shaped teeth. Hope its not a stupid Q!
Also would you replace the chainset rather than the rings? the XT chainset is only 129.
Also would you replace the chainset rather than the rings? the XT chainset is only 129.
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to many variables to say.
maybe.
Parktools :?:SheldonBrown
thanks for the advice, probably.