The duration is the length of time the entire RSS feed will be shown on screen - i.e. 900 seconds for all items. As far as I recall (it was 5 years ago) 1.2.2 does not have any way to select a number of items from a feed. Of course this functionality is in the latest version, and perhaps even a few versions before that.
I think you are left with 2 choices:
use a third party tool to pre-parse your feed and select the items you want (something on this list perhaps: Of course this functionality is in the latest version, and perhaps even a few versions before that.)
upgrade Xibo to a version that supports selecting N items from a feed (1.7.8 would be a good choice)