Say if there's someone you really want to be as good as.
A mode of thinking can be, Alright... every hour he spents drawing, I'll spend two hours.
Following that logic, say if you spent on drawing only 1/2 of the time that guy spents, and say he reaches a hypothetical mark in one year, you would at least have made the same progress in two years right?
That would be the case if devoting time is all that counts in improving your drawing skills.
What counts more than the amount of time spent, is the correct sequence of steps taken along the way. Say if you are building a pyramid with blocks, it would be obvious to build the foundation first. Thus the lack of foundation is the main reason why sometimes time devoted doesn't seem to correspond with the results.
Finding the best way to learn is alot more important than the time and effort spent.