mine's just a prefab deal, but it's about 6-7inches.
to be honest, I've never really sat at it, it was never intended to be a place to sit and eat/drink, it's a place to prepare food/drinks and then leave.
for my use, it's good. more of an overhang would just waste space(i don't have a lot to begin with).
if your custom building though, i would get the foot rail installed, pull up a chair and figure out what's comfortable, then grab a piece of wood/cardboard to be the top and slide it back and forth until you've found the overhang you like best. i'm sure the distance your foot rail extends will also greatly affect how much overhang you want. you don't want the top to be touching your stomach while your feet don't reach the foot rail