When you use play sand in a sandbox they recommend that you change the play sand in the sand box every 1 to 2 years depending on how often your child or children plays in the sand box.
I personally however do not see much reason to change the sand in the sand box because even like regular sand on beaches it never gets changed and is very old.
To keep the play sand clean enough and prevent cats and other animals from pooping in it or urinating in it put a cover on the sand box.
The cover will keep the sand in the sand box sanitary and I never have changed the sand in my child's sand box and it's the same sand 8 years later.
I see no reason to really change out the sand because it's just sand and dirty already.