I would hazard a guess it would depend on the type of composite structure and what loads it is carrying.
I know for a fact that Nomex honeycomb typically makes about 7-8mm thick composite for the skin (Lancair 360), but it was a stressed skin design with minimal longerons and other supporting structures with only three bulkhead structures along the inside (firewall, seat back, and one in the tailcone. These are always post cured/vacuum bagged.
Polystyrene foam sandwich is not something I am that familiar with, but I would imagine the thickness would vary depending on the load, and whether it is "cold layup" or post cured (baked)/vacuum bagged for better penetration of the epoxy or vinyl ester adhesive.