How does the military decide how of a bonus to give a soldier after reenlisting? much of a bonus?

My husband just reenlisted [ he's deployed] and he will be getting a bonus. He serves in the army in an airborne unit and is a specialist, been in for 3 years and he is a water purification specialist. He hasn't gone to college since he joined right after high-school. I'm asking because I want to make sure he gets what he's entitled for. He doesn't like to ask questions and I do. Any general overview is appreciated.