Hi Adam,
I'm surprised that the barns are only 15 yrs old. Who would build for longevity only to take down after 15 years?
As far as the time to disassemble:
Have you inspected to see how it was raised? This will give you the insight as to how it will come apart. Let you know how to brace as well; should be the same as the raising going backwards.
Being 15 yrs old you should not have to much corroded nails to deal with or heads popping off for the siding. However, you might be dealing with those damn nails that get fired through a gun with adhesive on them. Dunno. Nailing patterns are good to know as well.
As far as knocking pegs out. Piece of Iron with a slightly smaller Dia works well along side a hammer with some ass (oops I mean "mass").
With a plan grid you can come up with a labeling system that will benefit taking apart and storing. Big Red Crayons work real nice.
Other than that you could use some variables such as "X" time for work that can be done with secured footing and "X(1.5)" for any work in the air be it ladders or booms.
Hope this helps some.
