How long should security update 2015 take. Started update install and screen has been grey for over an hour. Something wrong?

Received notification for security update 2015, downloaded and restarted on mid 2013 MacBook Pro 10.9.5. Screen has been gray for over an hour, with no notifications.

Restart window said do not unplug power supply or attempt to shutdown during install. But this seems to be taking far too long, even for a large (150+/- mb) update. Anyone have a similar problem or have an idea of how long it should take for the install? Thanks for your help.