They say the average is 50 gallons per day. The thing is, they eat all of the suspended solids in the water, be that floculated gunk, plankton, etc. They don't actually "absorb" protein or nitrates from the water. Personally, I'd rather have the flocs and plankton in my tank water and use the skimmer to remove disolved organics.
David Eakin had 120 oyster reef tank set up at his house for a while and the oysters did pretty well. I'll have to ask him how long they lasted.
Cheers
James