Each block of the timeline represents 1 second. When you start a new project its current length is just 1 second until you start adding information.
Simply start writing your script and you will see the video length increasing with more empty blocks being added to accommodate your script.
To start adding visuals to your written store, drag any clip from the library down to the timeline. The default length of a clip that is added to the timeline is 4 seconds long. This is a good standard length to allow for enough time for the viewer to see the scene before moving onto the next.
To remove a clip from the timeline hit the backspace key. Or hover over the clip in the timeline and click the trashcan icon.
All clips in the library have a maximum length of 10 seconds so you are able to both lengthen and shorten any clip added to the timeline.
To change the length of a clip, hover over the yellow handles on the right and left side of the clip and and click and drag to change its length.
To edit what section of the 10 seconds are displayed on a clip see the Clip edit help guide here: https://intercom.help/adme-ab/en/articles/5615340-timeline-clip-edit-basics