When to COMPLETELY change and clean your cat litter tray?

I scoop the litter box every day and add 1-2 cups of litter each time. It is very clean, but I am not sure how often to completely change the litter. I have heard everything from once a week to every couple of weeks, and even just a few times a year (my vet suggested this and said to bleach it too). He said it is fine as long as I scoop daily and fully clean it a few times a year. I want to get more opinions on this.