With granite, I wouldn’t push it, 4 inch overlap should be fine for 1.5″ to 2 ” thick granite. You can go further, but bring the support layer out over the edge too. The support layer should be 3/4″ thick plywood and generally match the shape of your complete slab. The key is to properly distribute the weight, just like a waterbed.
Rule of thumb: If you ever question the over hang, do what I do…measure it with your hand at every bar you visit. It’s usually no more than a hands length.