If there is lots of algae in the tank, you can offer cucumber, or pumpkin once or twice a week. If there is no algae then you should feed them once a day.
Catfish, including algae eaters are normally most active after dark. So a lot of people feed them at night and remove the leftover food the following morning. If there are lots of floating plants in the tank the catfish might come out during the day to feed, in which case you can feed them during the day while the tank lights are on.