When you send money to the wrong number, immediately forward that confirmation message to 456. Note that here are no charges applied when using the SMS code 456.
Safaricom will respond in less than five minutes, telling you they have received your message, and have started working on it. Depending on the time of the day, sometimes it only takes 2 hours to reverse the money.
However, when it late at night you will have to wait until the next day morning, although you should still send the transaction message to 456.