Below the daily average chart, select the "App timers" option. You'll then see a screen where you can click on the different apps you have downloaded. Tap on the app you want to limit. Once again you'll see your daily average on that specific app.
At the bottom of the individual app chart, toggle the timer icon (hourglass) and set a time limit.
Tip: I usually allot an hour. I find it gives just the right amount of social media interaction throughout the day.
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Repeat the process to set a timer for another app. While it might be a pain to individually set timers as opposed to group apps together, it's nice if you want to set different time limits.