This is bad advice for people wanting to learn a musical instrument. It takes way more than 20 hours because much of what you do when you play say the mandolin has to become automatic in order for you to do it well. There are no shortcuts.
This is amazingly good advice for those learning their first musical instrument. Because casual observation tells me that nearly no one spends that kind of time on an instrument starting out. If one spends 45 minutes a day practicing (not noodling) on an instrument for a month, I don’t think sounding reasonably decent on a tiny set of tunes can be avoided. Rhonda Vincent won’t be inviting you on stage for a mandolin duet, but you won’t embarrass yourself in front of friends.
Now, getting good? Well, yeah, that’s going to be that same 45 minutes a day for the next ten years. But after twenty hours I think one would earn the right to call themselves a mandolin player.
I would say after 20 hours is a good sample size and that you should have a good indication on -
1. Where your skill level really is
2. What your current aptitude for learning said skill
3. How you feel about learning the skill and potentially going forward
4. Where you would like to go next to further push that skill
From there you may revise the course that you're on (add or reduce the time invested).
For me, I do like having 45 mins chunk of time to work through a problem. Its give me time to understand the goal, and get invested. 45 mins tends to fly by so you're not fatigued afterwards. Then you can rest and reflect.
That's how I'm handling my trumpet work (learning scales and songs).
Spending every single day learning something is the exact opposite of a shortcut, it requires a huge amount of discipline. 20 hours spread over 30 days should be more than enough to turn someone who doesn't know what a mandolin is into a novice mandolinist who knows what further steps he has to take to get better.