How to Use the iPhone Timer
You can set the time on your iPhone timer from one minute up to 23 hours and 59 minutes, after which, you're better off setting an alarm. After you set the timer, you can go on to do things with other apps, even press the Sleep/Wake button. The timer continues to countdown in the background and sounds when the time's up. To set the timer:
Tap Clock on the Home screen.
Tap the Timer button in the lower right corner.
Turn the rotor to set the length of time you want to pass before the timer sounds.
Tap When Timer Ends to choose the time's up sound.
Scroll through the When Timer Ends list and tap the sound you want. Tap Buy More Tones at the top of the list to go to iTunes and buy additional sounds.
Tap the Set button in the upper right corner.
Tap the green Start button on the Timer screen.
Tap Pause to interrupt the timer and then Resume to restart it or tap Cancel to turn off the Timer.
To use the Timer as a Sleep Timer while you're listening to audio or watching a video:
Tap When Timer Ends.
Scroll to the very bottom of the screen and tap Stop Playing.
The Time screen appears.
Turn the rotor to set the length of time you want to enjoy your media.
The countdown begins.
Click the Home button.
Tap Music, Podcasts, or Videos depending on the type of media you want to listen to or watch.
Tap your selection.
Tap the Play button to begin playback.
Whatever you are listening to or watching is turned off when the timer stops.