HT3576: iOS: Understanding notificationsLearn about iOS: Understanding notifications
How long it takes depends on which update you are installing and the speed of your Internet connection. Be sure your phone is charged up and connected to the charger or iTunes on your computer when doing the update. Also if doing the update while connected to iTunes on your computer it is best to disable virus protection and the firewall on your computer. Turn these back on when you are done.
Also be sure to do a backup of your device effort updating,