When I need them.
I've got a large drawer full of t-shirts, two pairs of non-tatty jeans and a few hoodies & sweaters which all get replaced when they start to look a bit tatty, which is probably every other year or so. I get a couple of workshirts a year and a couple of pairs of trousers. Anything else is just ah-hoc if something takes my fancy, like a casual shirt or bit of tailoring, but not very often.
I probably spend less than £500 a year unless I need to buy a new suit.