You should backwash whenever the pressure rises 8-10psi over clean filter pressure or you notice diminished flow from the return.

Lets do a test to make sure your sand filter is working properly. Buy the smallest bag of DE that you can find. While the pump is running, throw a handful in the skimmer and watch the return. If you see white powder shooting out of the return jet, we'll know your filter is not working. If you don't see it, it actually may help you filter out stuff a little better.


Read this for more info about that: http://pool9.net/de-test/