Once or twice a month should normally do the trick, but weekly disinfecting would help more should you notice sick birds at your feeders. Use good food. Discard food that smells musty, is wet, looks moldy, or has fungus growing on it. Disinfect any storage container that holds spoiled food and any scoops used to fill feeders. Keep rodents out of stored food.
Mice can spread some bird diseases without being infected themselves.
Don't wait until you see sick or dead birds. With good prevention you'll seldom find ailing birds at your feeders. Spread the word. Encourage your neighbors who feed birds to take these precautions as well. Remember that birds normally move among feeders and can spread diseases as they go.