I think it would have to be 3 feet for me to be snowed in. I've never seen 3 feet. I have seen 24 inches and that didn't stop me, so at least over 2 feet before I would call it snowed in.
8 inches of snow? I don't even bother with firing up the snowblower until 8 inches. 12-18 inches, now that is snowblowing fun. I still think I could get through 18 inches with my Jetta, so the snowblowing still isn't entirely necessary.