Mindy -
I think that, so far, you could easily get away with 4 full days for Halloween Time. I say "so far" because I don't know if anything new will be added in to the roster of seasonal fun within the next year or two that might require more time. Five days wouldn't be bad, if you can swing it, but 4 would be okay.
I think 8 days in December is a wise choice. Even though any days during the holiday season - 1 day, 2 days, 3 days, whatever - is better than nothing, as you know (because you've seen it), there is so much to see and take in during the Christmas season that the more time you can manage for it, the better, I think. Even doing something like getting a freshly made candy cane is a big deal and requires perfect timing - arriving at the Candy Palace early, waiting in line and getting a ticket, then coming back to get the fresh candy cane, etc. That takes a chunk out of the morning!