It depends on the species. I believe grammas are open-water feeders, meaning they'll only pluck food out of the water column and won't pick it up from a surface once it lands there, but I could be wrong; it's been a few years since my last gramma. That's one of the challenges when feeding with pellets and why most packages will tell you to feed "as much as your fish will consume in two minutes."
Typically when I've fed pellets, I'll introduce a little to the tank and watch the fish as they eat. Once they've eaten most of what's in there, I'll add some more, until I determine that feeding time is done. Don't get in the habit of just tossing food in and leaving until you have a better feel for how your fish feed and what their habits and needs are.
