The clocks, verbatim, because they are short and everyone gets one of them wrong. UCC 4-403(b): "A stop-payment order is effective for six months, but it lapses after 14 calendar days if the original order was oral and was not confirmed in a record within that period. A stop-payment order may be renewed for additional six-month periods by a record given to the bank within a period during which the stop-payment order is effective." Three consequences follow. A phone call alone buys two weeks. Six months is the outer limit of a written order rather than a permanent one. And a renewal has to be given while the order is still live, so an order allowed to lapse cannot be revived by a late renewal.
A different six months sits in the next section, and conflating them is the common error. UCC 4-404 provides that a bank is under no obligation to pay a check presented more than six months after its date. That is the stale-check rule, it runs from the date on the check, and it is a permission for the bank rather than a protection for the customer. Section 4-403(b)'s six months runs from the stop-payment order and governs how long the order binds the bank. Two adjacent sections, two six-month periods, two different start dates, and no relationship between them.
What happens when the bank pays over the order is the part worth reading twice. Section 4-403(c) puts the burden of establishing "the fact and amount of loss resulting from the payment of an item contrary to a stop-payment order or order to close an account" on the customer, and adds that the loss "may include damages for dishonor of subsequent items under Section 4-402". So a customer whose stop payment failed does not simply get the amount credited back. They have to show what they lost, which is a different and harder thing than showing that the order was ignored.
And then the provision that makes a stop payment something other than a refund. UCC 4-407 provides that where a payor bank has paid an item over a stop-payment order, "to prevent unjust enrichment and only to the extent necessary to prevent loss to the bank by reason of its payment of the item, the payor bank is subrogated to the rights (1) of any holder in due course on the item against the drawer or maker; (2) of the payee or any other holder of the item against the drawer or maker either on the item or under the transaction out of which the item arose; and (3) of the drawer or maker against the payee or any other holder of the item with respect to the transaction out of which the item arose." In plain terms, the bank can stand in the shoes of whoever was entitled to the money to the extent that the customer genuinely owed it. So where the underlying debt was real, the customer does not recover it from the bank; the two clauses limiting the subrogation, unjust enrichment and the bank's own loss, are what keep it from going further than that. What follows from that is the fact most worth carrying away: stopping a check does not cancel the obligation the check was written to satisfy. It is a brake on a payment, not a defense to a debt.
Checks and preauthorized electronic transfers run on separate systems, and the clocks differ. Everything above is check law. Where the payment is a preauthorized electronic fund transfer rather than a check, Regulation E gives a separate route at 12 CFR 1005.10(c)(1), under which the consumer notifies their own financial institution at least three business days before the scheduled date. That regime, its own oral-notice rule and the revocation machinery around it are covered on the ACH transfer and savings automation pages. Reading either clock as though it governed both payment types is the error to avoid, and the reason to know which one applies is that one is measured in business days before a scheduled date and the other in what the bank has already done with a physical item.
One note on the price. The official commentary to Regulation DD lists "stop-payment fees and fees associated with checks returned unpaid" among the charges that are not maintenance or activity fees, which means an account advertised as free can carry a stop-payment fee. That list, and what it does to the word "free", is covered on the maintenance fee page.