How long does it take to reshingle and remove 2-3 layers of shingles on a 28 square house?

By yourself? 2 people? 5 people? It is a labor intensive work shovelling off that much shingles. 2-3 days with 3-4 experienced people using air nailers, working steadily and 1 cleanup person on the ground.

2 layers are 33% less than 3 layers, so which is it? Also, what are the roof boards made of, and how rotten might they be in places that need would be to be replaced? A time estimate can't cover that, unless they can examine the attic, and even then, if nails are falling out of the wood on the outside it may be not visible inside.

And of course after shingles are taken off, how much of the under boards are bad and will need to be replaced