i just passed by a Sports Basement store near me, outside, alongside the display of a Cannondale cross bike, i saw an employee's hybrid with a quill stem. it had about 4-5 inches of stem showing above the top of the headtube and after a 45 degree bend, it had another 4-5 inches to the handlebar clamp. i often pass by the Rivendell store and some of their bikes have even more stem showing.

in reality, there can be no useful rule of thumb, IMO. there are just too many variables.

on the stem itself: overall length, percentage of overall length in the headtube, angle of bend, clamp type, material, diameter, anchoring mechanism, percentage of clamping portion to insertion portion...

other non-stem factors: weight of rider, type of terrain, riding technique, average speed...