Depends on the style, tbh.
when it was short, it was every 4 to 5 weeks.
At the moment I am growing my grey out (my hair is to my shoulders) so only having a trim when it’s looking untidy. Had it cut in august and planning another for December.
then probably leave it til my birthday in April and have go back short which will cut the colour out and leave me with just grey. Then grow it again.