How long would a cancelled note (regardless of technique) remain in Treasury hands before being eventually destroyed, if that was in fact the case?
I didn’t find anything specific to cancelled US bank notes, but Howard M. Berlin’s book The Coins and Banknotes of Palestine Under the British Mandate, 1927-1947 gives some insight into the subject through Palestinian Mandate currency.
According to Berlin, “Even as late as 1974 bank notes of the Palestinian Currency Board were redeemed and the Crown Agents in London still await any bank notes that may be redeemed