1. Will the Timer help?
Clock -> Menu -> Timer -> Menu -> Set Timer
Then set the timer to 15 minutes
Menu -> Start
When the timer goes you restart it again
Alternatively
2. Will a New Alarm plus Snoozing help?
Clock -> Menu -> Options -> Snooze Time
Set Snooze Time to 15 minutes then: Menu -> Save -> Escape
Calendar -> Menu -> New Alarm
Set a New Alarm to start immediately.
When the Alarm goes keep on snoozing.