How Often Do You Clean out the Nesting Boxes?

I have hay in my nesting boxes and once the hens made "nest" of it, none have rearranged the hay. It remains clean as well and the eggs are clean. They have been using the nesting boxes just over a month. When should the hay be removed and new nesting material put in?

I will change it if the hens poop in it or if the eggs are getting dirty. I also change it when I treat the birds for body parasites. Often I will just add more if it is getting a little thin or kicked out.

Because I use shavings I don't really have to 'change' it. It stays very clean and when it starts to get a little thin I just top it up.
I really just don't have a problem with dirty nest boxes or dirty eggs.