Did a little googling for you. Most results seem to say about 1 to 1.5 inches. Another says to measure the thickness of the hearthstone and match the overhang to that. It doesn't seem there is any true standard.

There are no standards, just consistency. My recommendation is 1 1/4".

The pointed end is even more than 1 1/2" beyond the face of the vertical stone, maybe as much as 2 1/2" or 3" at the tip. We'll have to cut it off "someday." Hopefully before it cracks when a 300 pounder sits on it! Our temporary solution is to place a plant in front of the ends.

Anyway, the rest of the hearthstones and grout are stable. We put a galvanized nail in the grout / mortar of the overhangs to strengthen it. Learned that trick on Google. Also tuck pointing with a little tool from the local box store. Thanks for the suggestion from KP.

The mortar turned a soft gray/ash color after it dried out (not shown in the pictures), matching the surface of the hearthstones. Looks good.

Time now to start working up the sides and top of the fire box with the Versetta Stone panels. Will have a few design questions later about that.