The Timer application can be used to alert you with an audible signal when a set amount of time has passed.
Open the Alarm/Clock application and tap the Timer tab.
Set the time by scrolling Hour, Minute and/or Second on the time wheel.
Tap Vibration to checkmark the setting and have the phone vibrate when the set time expires.
Tap Alert sound to select an alert tone for the timer, then tap OK.
Tap Start to begin the time countdown.
Once the alarm sounds, tap Stop to stop the timer.