This is a question we get asked OFTEN by do - it - yourselfers.   The answer to this question is when the pressure on the gauge rehashes 10 psi above where it normally runs when the filter is clean.   SO, this also means, make sure those pressure gauges are working so you can monitor the pressure to know when backwashingnis needed.  There are a few other ways to know when backwashing is needed also, but we will save that for a later blog post.  ENJOY those pools folks!!!!