"Not very much" - if you over-torque them you may end up with a pulled or snapped stud, which is repairable but a royal pain.
Haynes manual shows from 22 to 26Nm, or 16 to 18 lb-feet. I certainly wouldn't go over this amount, and possibly a bit less. you just don't want it to fall off or vibrate loose or for the gasket to leak oil. If you find yourself cranking down on it
to try to stop an oil drip from the rocker covers, it is time to get a new gasket for the rocker cover. Basically I treat these
sort of like sparkplugs - you want them snug, but not very tight.
