I would let the anemone be your guide. Watch how it reacts when you feed your fish; you'll learn to recognize when it's looking for food, because its mouth will look puffy or puckered. If you place a small bit of food on the tentacles, a hungry anemone will grab it and consume it pretty quickly.
I feed mine small, thawed pieces of krill or scallop when they look hungry, which works out to about 3-4 times a week. They've been doing very well and growing steadily (you can see them in my avatar). If they aren't hungry, they simply won't take the food--just make sure to remove the rejected food (if your other animals don't eat it) so it doesn't start to decay in the tank.