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.
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).