how can I reduce the time my alarms ring for

I don't believe there is a way to limit the alarm time other than pressing Stop.

Hello, take a look here:

To change length of alarm…

In the Alarm tab of the Clock app, either add a new alarm with the "+" button or hit "Edit" and select the alarm you want to change. On the edit screen, make sure "Snooze" is disabled, then set all of your alarms 5 minutes apart (or whatever time you want)

Also….

To stop alarm from repeating…

Open Clock and tap “Alarm” Tap Edit. Tap your alarm that repeats and set the repeat to “Never” Save.

Also…

How do I make my alarms last longer or shorter…

Open the Clock app, tap the three-dot menu button in the top-right corner of the screen, tap Setting > Gradually increase volume (under the Alarms heading), then pick a setting—anything from five seconds to a full minute.

~Katana-San~