I would spot clean everyday - if I see a poo then I pick it up
Alan is right - it's all good if they do their business in the bath, less mess in the enclosure.
I also agree with him in that the bigger the enclosure the longer you could leave it.
With a small enclosure I'd do it every week/ couple of weeks depending on how much mess is in there.
Sand and soil should be mixed to try and replicate a more natural substrate and to avoid possible impaction from the sand