Sitting for long periods in the cold is miserable. I have to agree with Greg though on the time frame. I can sit about two hours, maybe three if there is no wind and the temp is not too bad.

My feet and hands tend to get cold first. For this issue I carry heat packs, and put one in each boot not too long after I get there. Right under the flat part of the foot is best for me, and I lace my boots loosley.

I have also found that coverall-style camo is best for cold. Don't get me wrong, I much prefer bibs and a coat for mobility and certainly at work, but the coveralls (particularly if a little baggy in the waist - this isn't a fashion show afterall) can be heated with a hand warmer also. Since your top and bottom are connected, a heat pack stuck in your belt will help generate some good upper body warmth as well.

I also use something called a bow bra. Yes a bra. Look this ain't no fashion show, and by the way, sometimes I wear my wifes panty hose which I cut off at the knees to keep my feet from blistering on a long hike. Anyway, the bow bra keeps the heavy clothes from getting caught in the bow string when you shoot. I got mine at a deer and turkey classic some 20 years ago, but I'm sure somebody still makes them.

Cold weather hunting for me is a lot like exercise, they both suck, but look at the beauty and enjoyment you get from life having stuck it out.