If you are using a tub with paper towel or news paper, once a week. If you are using aspen or some other bedding, spot clean as needed and full clean every 3-4 weeks.
If you are using a tub with paper towel or news paper...
...every time your snake makes a mess. For some that's once a week, I've had others that have had to be cleaned twice a day, but unlike my retics they usually don't make any more messes for a while.