Answer :
-- Cut the sheet in half.
-- Put one half-sheet on top of the other half-sheet.
Cut the stack of 2 halves in half.
-- Put one stack of two quarter-sheets on top of the other stack of quarter-sheets.
Cut the stack of 4 quarters in half.
Now you have eight one-eighth sheets.
Three cuts.
Taking Sam (3 x 5) = 15 minutes, plus a few seconds for set-up between cuts.