I have one russian tortoise, I have two pretty big elcosures that are ramped so he can walk to each. One is about 6ft by 3ft and the other is 5 feet by 3 feet. I use eco earth and spot clean whenever my tortoise poops or pees. How often do I need to completely change the substrate? Since I have 2 enclosures that are used it is pretty spendy (it will cost me like $50-$75) to change.