At least a week, probably much longer. A half teaspoon or so of "shrimp soup" every day or two gives the fish an agreeable treat without excessive nourishment. If fed all at once, most of a frozen cube would just land on the bottom and contribute to the litter.
I put the cube in a cup of hot tap water (non-chlorinated) and then feed them by hand with a toothpick, just because I like the interaction. I have heard of feeding cones for bloodworms, but I wasn't able to find one to show you......