depends on what thier set up is. if it has an established system they can live a long time. mine went months on eating baby guppys and whatever else they could find
My brother puts crickets in his tank once a week. Of course they don't all get eaten on the first day but if its a matter of how often to feed, once a week should be fine.
mine pig out every week if i go a bit longer they find a cool spot under the moss till they get fed again just make sure they get a varied diet earth worms silk worms meal worms wax worms, crickets ......