Depends on what kind of hunting and what kind of service you are pressing it into. Generally saying 'hunting knife' has a fairly broad interpretation IMO.
If you are using it simply to field dress and skin out deer or smaller game, you can go quite thin. If you are gonna quarter something larger, or try to take a wild boar with blade alone, you might want something thicker...
Like Shaw said, as thin as you can go without deformation. Thin wins...
Cheers.