Dude,
Our standard recommendation is to backwash, or clean your filter, whenever the pressure has increased by 25% over the clean pressure... This is a general rule and is designed in an attempt to cover all kinds of pools with all kinds of plumbing.
I think it is a good place to start, but each pool is different and you are really the best judge of when to clean your filter..
Keep in mind that backwashing does not remove all the old and dirty DE only about 80%.. I don't advise backwashing more then twice before cleaning the filter...
Thanks,
Jim R.