business/commercial law

Sam is sitting at home when some teenagers knock on the door and offer to rake his yard for $30.00. He agrees, writes them a check for the amount, and then leaves to go to lunch. Upon his return, he discovers that they took his check and left without raking the yard. Sam calls his bank and requests that they stop payment on the check, which they agree to do. How long is the stop payment order valid? Would it have made a difference if he had written the bank instead of calling?