Let's say your 10-year-old son has a natural aptitude for the piano. You put him in lessons and he does well but after a while he gets sick of practicing.
As his parent, you make a deal:
He has to practice every day until his next recital. After that performance, he can quit if he wants to.
Here's the question...
How do you define how much he needs to practice each day leading up to the recital?
I see two obvious approaches:
I guarantee you that the results-based approach will create a better piano player because it focuses on the desired outcome.
The time-based approach is a causal fallacy that encourages gaming of the system.
A professional piano player is not compensated based on the number of hours he or she spent practicing. They are compensated based on how effective they are at moving an audience.
Here's the thing...
Your clients don't care how many hours you put in. They only care how effective you are.