You use it as often as you need to.
For instance if you live next to a railway line, then maybe every month, if your car lives in a garage and dosnt get driven in the wet, then once a year.
Have a feel of the car (back of hand) and if it feels rough, bit like sand paper, then it needs doing.
Once clayed then a decent wax or sealant on top. If you dont have that much cash use Carlack from Tim at CleanYourCar.
I use Bilt-Hamber soft clay with ValetPro Citrus Bling as the lube. Works for me.