It really depends on your personal skin chemistry, if you're asking how often you should do it to maintain soft skin. If you have especially dry, flaky skin that often forms 'scales' it should be fine to do it every other day, but if you have easily irritated skin that is more oily than flaky you may want to stick to twice a week.
Now how often SHOULD we use a body scrub? I'm an advocate of not disturbing your skin's natural flora and fauna unless I have to. Because of this I scrub my arms every week, use acne face products, and wash my hands often. Often excessive oil/dryness is because you have disturbed your natural fauna with daily body wash, so I try not to wash daily with anything other than water. It's something that a lot of dermatologists debate, though.