It varies with the gun design, but .06-.07" would be a good average. It doesn't matter if it is a bit too long if it doesn't protrude and either interfere with feeding or cause the gun to fire when the bolt/breech closes.
Just FWIW, too much protrusion does NOT cause "pierced" primers. Unless the firing pin has a needle nose, no firing pin spring ever made has enough power to actually pierce the primer. So-called "pierced" primers are caused by too light a firing pin or firing pin spring, when the pressure forces the firing pin back and pushes a disc of primer metal back into the firing pin hole.
Jim
