scafool, I recently had the opportunity to gather hard data on this. A windstorm took out the 15KV supply to my parents' house while they were on holiday; I was on duty as Generator Man for the better part of three days.

Horizontal (chest) freezers:

If packed dead full, in a cool location (basement) will last for 24 hours plus. If you don't open the lid, and pack blankets around them (unplug to avoid fire hazard upon startup), you could probably push that by another 24. You might sacrifice food quality by mild defrosting, but I don't think food safety would be compromised. Though I would keep a close eye on seafood for example.

If not packed full, 8-12 hours is the best you'll get. I was getting the first signs of thawing in a 1/3 full freezer that held freshly processes garden vegetables. It helps a lot to consolidate the food at the bottom, drop in whatever ice you have, and pack a blanket in there (or even lots of sheets of newspaper); anything that's a barrier to air movement is helpful. I spread out a blanket on top as well; hard to tell how much that mattered, though it couldn't hurt.

Vertical fridge/freezers:

These are generally much worse. 12 hours max. if it's packed full of hard-frozen blocks, and as little as 6 hours if it's small odds and ends.

I had to stick a sign on the old vertical side-by-side fridge/freezer ... "freezer has partially thawed, don't trust it." Everything stayed at fridge temperatures, and could be cooked and eaten right away; but refreezing some items (pork, seafood) is asking for trouble AFAIK.

If I had packed in extra ice and blankets (or crumpled newspaper I suppose) I could have stretched out the timeline, but by the time I got there the deed was done.

Bottom line:
In all cases: ice and insulation are your friends, and open air space is your enemy.