A few times a month. Generally speaking they will eat squid, shrimp, clam, and fish but some are picky so you will have to try a few things. You'll also find that sometimes they will like something once but never again. It may even eat one of your tank residents so you won't need to feed it that week
Feed it small pieces instead of a whole shrimp or silverside. That is, feed it the whole shrimp/fish but cut it into smaller pieces.