APSP-15/NSF 50 standards are as follows:

Cartridge: 0.375 sqft/GPM
Sand: 15 sqft/GPM
DE: 2 sqft/GPM

The same standards suggest using a 6 hour turnover as the flow rate. Personally, I think you are better off sizing the pump for 8 hours and the filter for a 4 hour turnover. That way you have a correctly sized pump and an over sized filter which is a good thing.