Everyday for me- except Sunday usually. Philip Kingsley argues that you should treat the skin on your scalp as you do on the rest of your body- although not to the extent of washing it twice a day, etc.
I’ve spent a fortune on hair products over the years and find having several shampoos and conditioners to choose from depending how my hair feels on a particular day helpful. For instance on PMT days when it’s greasy I use something clarifying and a lighter conditioner, and alternate between colour protection and moisture on other days.
It works for me.
Also (as I’ve mentioned before on a similar thread), apparently Olivia Palmero also washes her hair everyday and her hair is gorgeous…,unfortunately my hair isn’t quite like hers, but maybe, one day
!