Chances are the shortest interval you can set between alarms on your device’s built-in clock, calendar, or reminder app is 24 hours. But there are some haphazard workarounds.
On Android, you can tweak your phone’s snooze duration to have an alarm sound as often as every 30 minutes. To do this, go to the Clock app and set up a regular alarm by tapping the big plus button at the bottom and adding your desired pa
rameters. When you’re done, tap the three dots at the top right corner of your screen and go to Settings. Scroll down, find Alarms, and tap Snooze length. There, choose the last option, 30 minutes, which is the longest time you can snooze an alarm for. Keep in mind that Android’s features differ greatly depending on your device’s manufacturer, so don’t be surprised if this option doesn’t exist or if you can actually set up recurrent alarms (lucky you).
If you do have to use this strategy, hit snooze every time the alarm goes off and you’ll technically get alerts every half an hour. Other than the fact that this is a limited trick, the biggest downside is that because the snooze becomes the actual alarm, you’ll have to do the task right then and there, or risk forgetting about it until the alarm goes off again.
Apple doesn’t let iOS users change its default 9-minute snooze duration, so you can’t use this trick if you have an iPhone. What you can do is set a timer in the Clock app and hit Repeat from your phone’s lock screen every time it goes off. But this only works if your phone is locked when the alert hits. If it’s not, your best bet is to open the Clock app and simply set a new timer. This, of course, is barely automated and less than ideal.
But there are third-party apps up for the task.
Hourly Chime may be one of the simplest recurrent alarm apps available. All it does is offer alarms that’ll go off every hour, on the hour. But you can choose which alerts sound on which days and add notes to each one so you know what you have to do every time. What you can’t do is edit the times. Hourly Chime hasn’t been updated in five years, but considering how basic it is, this should not be a problem.
If you want more settings and options, Repeat Timer is a more complete alternative. Reminders can go off as frequently as every second (why would you do that?!) or as far apart as an entire day. A basic account will get you only five repeats for a single timer, but you can set a new timer to repeat five more times when that runs out. For $5 you can get 99 repeats per timer, plus 35 more sounds and the ability to have up to 10 timers running simultaneously.
Source : https://www.popsci.com/diy/recurrent-alarm-apps/