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 touch the Timer tab.
Set the time by scrolling Hour, Minute and/or Second on the time wheel.
Touch Vibrate to checkmark the setting and have the phone vibrate when the set time expires.
Touch Alert Tone to select an alert tone for the timer, then touch OK.
Touch Start to begin the time countdown.
Once the alarm sounds, touch Stop to stop the timer.