Default duration on Modules page - that’s a default duration for that particular widget, so when you add a new widget to your layout and don’t set the duration by yourself it will use the default duration specified in the module.
If you set the duration on the widget itself then it will use that duration instead.
15s for tickers (I assume) might be fine, depending if you also have duration is per item enabled or not and how many items are returned in those feeds.
Perhaps have a look at this topic about durations - Understanding Media Duration and how it affects the playback of a Layout
Unless there is some other issue on your layout, I’d guess that the black screen you’re seeing is the layout reloading, so you could increase the duration of your widget or add another widget with longed duration if the 15s/ticker works well with the content they return.