It should be noted that, per the original question, that square post has a conical chamfer machined into it; actual wear will be evident by a slope in the top (circular) face that the cam interfaces with. It is square to the sides of the post when new...will show sort of a "trough" as the cam begins to wear the middle, and will eventually show a gently curved slope.... I would guess that as long as you can still see some of that original chamfer, there is enough stem length to actuate the valve.

You are likely to have functional problems from dry/cracked sealing gaskets (failing to fully CLOSE) long before that brass post wears down enough for failure to open.... And in my experience, while that post technically unthreads to allow replacement of the gasket, you are much more likely to break the threads off then successfully remove the post, and will end up replacing the entire plunger assembly anyway.