Comment 1 for bug 490065

It is a deliberate design-decision to not offer applications control over the expire-timeout to help avoiding cluttering the desktop with notifications. It works in tandem with other spam-protection mechanisms in notify-osd. This prevents these situations from happening:

http://www.futurecrue.com/wp-content/uploads/2009/07/notification-daemon.jpg
http://urchin.earth.li/~jds/twitter-linux.jpg
http://3.bp.blogspot.com/_IWryv4TJmYk/SFHbL--EEWI/AAAAAAAAAEQ/ke9HSqN858M/s320/TwitterPop.jpg

Please also see https://wiki.ubuntu.com/NotifyOSD#org.freedesktop.Notifications.Notify for further details on this topic.