By default, Mac OS X displays notification banners just for a few short seconds. Here's a quick way to change how long the Notification Center banners should be shown before they disappear.
The prefs value responsible for the display time is
defaults write com.apple.notificationcenterui bannerTime -int SECONDS
The system default value can then be restored by typing:
defaults delete com.apple.notificationcenterui bannerTime
Original source of solution is here.