Duration is set per item, in the playlist rather than on the content, so the same image can run for five seconds in one playlist and thirty in another.
Defaults and limits
An item defaults to 10 seconds. You can set anything from 1 second to 24 hours.
Video and audio are the exceptions. They play for their own length, and the duration cannot be edited. If a video shows no duration at all, the upload did not finish processing: delete it and upload it again.
Choosing a duration
Ten seconds suits an image someone glances at in passing.
A dashboard needs longer, both to be read and to finish loading. A heavy dashboard may need 30 or 40 seconds before it has even rendered, and a page that is still loading when its turn ends never gets seen at all. If a web page looks blank on screen, try giving it more time before assuming it is broken.
Longer is not automatically better. A rotation where each item sits for a minute is a rotation most people never see the end of.