I was told this by a lady a few weeks ago. Very good indicator of how long your power has been out.
Up here in the PNW we lose power alot. I was given this tip to make sure that if you are gone you can tell if the contents of your freezer got too warm during a power outage.
Take a 12- 16 oz. water bottle, the kind with ridges in the side are preferable, but any will do. Fill to halfway, cap it and turn upside down. Put in freezer overnight. Next day turn back over in freezer. All frozen water will be in the top of the bottle. This will be your indicator. When you come back from vacation, look to see if the water is still in the top of the bottle. If it melted and the block fell to the bottom, the contents are questionable if not outright bad.