Change Notification Banner Display Duration
Notification Center announcements come either as banners or alerts. Alerts generally require some kind of user interaction, but banners slide in and out after a short three seconds - that's the default, that is, and you can change it.
Within System Preferences' Notifications panel you are allowed to change the way an app displays its announcements via Notification Center. Alerts are the old-fashioned way to go, making sure you don't miss the announcement by requiring user interaction to dismiss the alert itself. Banners, however, are the modern - and fast - way of transmitting information particles. Per default, they slide in and out within three seconds time. This may not be the ideal time frame for everybody, as it's rather easy to miss a banner or two, for example, when talking to a colleague "over the display".
Fortunately, it's rather easy to change the banner display duration.
You can wind back the changes by replacing the above defaults command with
We haven't tested if there's an upper limit to the number of seconds you can provide. (MacConfig)