By default all banners from the Notification Center stays 5 seconds on your screen. You can adjust how long Notification Center banners are shown before they disappear. To do this use the following command in OS X Terminal:
defaults write com.apple.notificationcenterui bannerTime SECONDS
Replace SECONDS with the number of seconds you wish the banner to display. Restart OS X before the changes take effect.
Restore the default setting:
defaults delete com.apple.notificationcenterui bannerTime