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 SECONDS
The system default value can then be restored by typing:
defaults delete com.apple.notificationcenterui bannerTime
Original source of solution is here.