How long does an iPhone alarm last?
The alarm will last about 15 minutes, according to Anna Fyen's answer on Quora:


  My alarm works fine every weekday. I cannot give you the exact time. But one day, I set an alarm and let it ring for a while so after about 15 minutes or so of ringing it turns itself off and displays a message on the lock screen.


Phoning an iPhone currently playing the alarm will likely interrupt the alarm and replace the sound with that of the ring tone.