How ofter do you have to clean a tarantula's tank?

Do not change the cage too often cause they get used to thier surroundings and it can be stressful if you change it too much. Do the spot cleanings like suggested above. Also let the substrate dry out completely every week or 2 to make sure not to let any thing breed in it. This reduces your chances of getting things like mites. 6 months to 1 year is my suggestion for cleaning it entirely as long as you keep it spot cleaned.