Is there a way to set up a schedule that dictated just by a length of time?

Is there a way to set up a schedule by a length of time say 20 minutes or 45 minutes? Rather than using the time of day… My schedule varies greatly, and sometimes I need to turn off access to apps, but it’s not at the same time every day.

Yes!
When you open the Opal app home screen, hit “start blocking session”. This will prompt you to pick a duration to start a session immediately.