The answer:
Here is an explanation from the archives that may help you work out this frequency reduction. Beyond this, you will have to research the ATEX Archives yourself for more details.
Editor
Reducing the cleaning frequency from 5 days a week to 2 will result in extended cleaning times. After all, the customer has carefully and thoughtfully saved up soil just for you. There will be more ... — Gary Clipperton, president of National Pro Clean Corp.