[sreyaNotfilc]

I think every advice can be malleable. Just getting started and seeing some results (whether its 20 hours, or 2000 hours) is what matters.

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).