A tablespoon is the rough rule of thumb for Syrians
mine gets a slightly overfilled tablespoon each night.
Start with that, and observe their eating habits: if they eat everything and you can't find any stored food, feed them more. If you find they're hoarding a very large amount, feed less. You want to find the balance where they can eat their fill and still be able to store a bit of it away, but not too much as that can encourage over-eating
hope you find this useful!