[idealmedtech]

It’s under the new “digital well-being” menu in settings. Quickest way to get there is to tap and hold YouTube’s app icon, then click the little (I) or gear, which will take you to YouTube’s individual settings. One of them should be the app timer.

[MisterSandman]

I believe that's a Samsung-specifc feature, no?

[idealmedtech]

It’s available on stock android as well (I use a Pixel). Not sure about general availability.