You'll see the duration on the bottom of the order panel.
Most people don't worry about it too much because all it does is fine tune what happens to an order if it doesn't immediately execute. Leaving the order as 'permanent' - the default - gives the best chance of trading the order.
The duration option is a one-of-many selection:-
Your permanent order is matched as far as it can be, and what's left over is posted on the public order board until either it's completed, or it's cancelled by you, which is why it's sometimes called a 'good until cancelled' order.
Your immediate order attempts to execute what it can now, within your limit, and abandons the rest without posting it on the order board. An immediate order prevents your available resources being reserved against possible future settlements on open orders.
A 'fill or kill' must complete immediately in entirety, or be abandoned in entirety, so it never part executes, and it never goes on the order board.
The final option works like a permanent order but sets a time limit which says when the remnant of your order should be withdrawn on your behalf from the board - if it is still unexecuted. This is sometimes called a 'Good until' order.
When you select the option for 'Until date' the default is the end of the current day. If you click on that date the system will display a calendar, and allow you to select your cancellation date. Be sure your time-zone is correctly set when using this option.
