How long dose it take for fingers to prune in hot water?

Depends how hot the water is

3 years

Shouldnt be more than a few minutes. As soon as the power goes back on, within 2 minutes, you should hear the heating elements making a popping or clicking noise as they heat up.

Depending on the size. Usually about 2 hours though.

There is no "one" specific answer to this as it depends if it comes from an "on demand geyser", "gravity fed geyser" or "high pressure geyser" and the distance between the geyser and the tap