Depends on the size of the roach. The different life stages for the dubia roaches have a pretty vast size difference, so it'll depend on what size dubias you're working with. There's a rule that's something like as much as they'll eat in a certain number of minutes, but it's escaping me right now. Hopefully someone who remembers it will reply (you can also do a search on here, it's been mentioned in loads of threads)