When panels are initially created, their default length is 1 second, displayed in timecode format. When it comes to your animatic, you will want to be more accurate. Modifying the duration of a particular panel is key when working out timing. There are a few different ways to accomplish this.
In the Timeline view, it is easy to change the duration of a panel just by dragging to resize it. This way, you can easily see the length of panels in relation to one another.