[emma_mcraf]

If you maintain the soil regularly by poop-picking daily, turning it over to keep it fresh, soft and moist but not wet, then I think about every quarter. I've read people doing it every month, but I don't think it's necessary if it's maintained. I often get a trowel, and my hands and just give it a good mix. If the water bowls are wet and not just a swamp, I empty them into the table and mix in with the soil each morning.
In the summer, when torts are outside more and not using their tables so much, I don't tend to change the soil at all. I might top-up with fresh topsoil and mix it in.