I'm not sure there is a good answer to this question. I think it depends on the species of clam. I think larger, deeper water clams like derasa & hippopus require large quantities of filterable nutrients. The shallow clams that get more direct light and are in higher currents may require less.
I don't know if size of clam really is proportional to nutritional needs. The studies I have seen note about 70+% of the nutritional demands of the clam are met by it's mantle symbiotes creating storable energy for the clam. that implies that ~30% needs to come from somewhere else.
In a pristine system without feeding this may lead to slow starvation. Some people have had great success without feeding but there systems are all different(high bioload, inc nitrates etc...)
I recommend regular feedings every 3 days with a Live phytoplankton of small enough particle size to be utilized by the clam.
Adam