I made 90 out of 100 20-foot putts this morning in 12 minutes using 3 putters. There's a 3.5 minute video of McBeth making 50 putts plus messing around with a basketball (50 +1 on youtube.) If each putt takes 10 seconds, that's under 17 minutes for 100 putts. So 20 minutes is definitely realistic for 100 putts. Though I can see if you only use one putter, and go through a full routine of marking your lie, setting up, and putting each and every time, it would take much longer. Which would also be a great way to practice, since that's what happens when it counts. But for building muscle memory, confidence, "greasing the groove," etc., 15-20 minutes is definitely realistic.
Also, I must not have been clear enough on the point of "importance" since it looks like you misunderstood what I was saying. I am not equating importance with commitment. I am not saying that disc golf is not important to you; I am saying that putting practice is not an important focus. Which is fine; it sounds like you are choosing to focus practice time on upshots. There is only so much time, and priorities have to be made. So if you choose to spend disc golf practice time on upshots rather than putting, wouldn't that mean that you deem practicing upshots a more important use of time than practicing putting? Or for that matter, watching videos, commenting on message boards, daydreaming about your first ace, etc. must also be more important to you than practicing putting, since that is where you are spending your disc golf "time budget?"
Those other activities are certainly valuable, for learning, motivation, enjoyment, etc. What I'm saying is that we all prioritize what are really the important things first. So if putting practice is at the bottom of the list, then relative to the other activities, it isn't of high importance. Otherwise it would be at the top of the list.