When placing an order, you select whether you want the duration to be a Day order or GTC (good-till-cancelled).
A Day order will stay open until it is executed during the trading day, or until the relevant market closes. If the order is not executed by the end of the trading day of the relevant market, the order will be deleted automatically.
A GTC (Good-till-Cancel) order, on the other hand, will roll over into the next trading day if the order is not executed by the end of the trading day. The way GTC orders are handled will depend on the trading venue. For example, orders placed on Euronext will be valid for one year while orders placed on US exchanges will be valid for 90 days. However, there is no guarantee that the order will not be cancelled for other reasons such as a corporate action of the underlying company, or that the market price has moved too far from the set limit price of your order. For this reason, you should monitor the status of your open orders.