This is a common concern with avid gym enthusiasts because they are so used to spending hours in the gym for one workout.
Great news for you though!
Yes, a 20 minute workout is effective
In fact, you could argue (depending on how you train now) it could be even better.
I would argue the majority of people train too hard. They think more is better so dialing back the frequency and intensity and giving your body a chance to recover properly can do wonders.
What I suggest you do with this 20 minute upper body resistance band workout is use it as an extra workout on the weekends or use it to replace your upper body workout a couple of times a month (if desired).
You can honestly throw it in anywhere you want!