Depends on where you live.
Here I can dig a 4 foot hole with a post hole digger in less than a minute. You may need dynamite to go 6 inches.
I've learned that the deeper the better. I have foxes around here that will dig up anything dead I bury if its not at least 3 feet down. I've tried covering the hole with boards, blocks, etc.. only to have the fox dig in at an angle.