That depends greatly on what type of substrate is used, how many torts you keep in there and how often it is fouled. My tort has a soil substrate and rarely poops in it as he poops in his soaks daily. This keeps his substrate much cleaner meaning it will be changed out less often.
When I kept small tortoises inside I would spot clean as needed by scooping out and smoothing over or filling in the area.
But in over two years, I never did replace it all. It just wasn't needed.