When you call a broker or click the “execute” button on an online interface, your trade ultimately goes to an intermediary, who must determine to which market to send the order. The stock orders may or may not be packaged together with many other orders to be fulfilled at the most favorable price. According to the SEC, there are no current regulations making brokerage firms execute trades within a certain time, only that the firm cannot deceptively advertise its ability to trade, and must meet its promises to its clients.