I have a 3" N. Wennerae in a 6g Eclipse. During the summer when I can get small 1-2" crayfish, I offer him a new one as soon as he finishes the last. I make sure he is eager for the food. He'll eat 3 or 4 in a row over a week then lose interest.
During the winter I feed him snails, usually nassarius because they are cheap here. He can eat a half dozen a week, but I usually space them out quite a bit more than that. Say 1 or 2 a week.
There are many other things they will eat, this is just what I am in the habit of providing.
HTH
S !