Change of duration is possible for images and PDF pages.
For a multi page PDF, the duration will apply to each page indiviaully, so setting 10 seconds for a 5 page PDF, will show each page for 10 seconds.
Video items get their duration from the video length, and it is not possible to change them.
To change the duration, click on the properties dialog:
Change duration here:
Once you type a new number of seconds, you press enter or click anywhere else, the duration will get updated.