So, often piano pieces are graded by difficulty.
Like, for example here: http://www.henle.com/us/detail/index.html?Title=Two+Part+Inventions+BWV+772-786_169
The editor has to say:
While assessing the pieces, it became clear that the medium level of difficulty (4–6) is the trickiest. Now and again this means that a piece is judged as a "3/4", even if it only deserved a "3" as far as piano technique is concerned. An example of such a "borderline" case (easy/medium) is Schumann's "Scenes from Childhood" op. 15 Von fremden Ländern und Menschen or at the other end "6/7" part of Bach's "English Suites". And of course within a main category there are also "from-to" evaluations (e.g. 7/8).
Okay, but how much time is factored in the evaluation? How much time is reasonable?
How much time are you supposed to spend on a piece approximately at your difficulty level?
Are you supposed to sight read a piece to say it's "at your difficulty level"?
Is it customary to spend a couple of hours on a piece or a couple of weeks? Months?
Let me be clearer with an example (and please take it as such): the Two Part Inventions are graded as "level 3".
It's a subjective grade in a scale the series editor came up with, fine.
I've recently started playing piano again, and I picked up the Two Part Variations myself.
In two evenings of practice (say 4+4 hours?) I managed to play tolerably the first half of the first variation - I'll tackle the second half tonight.
Does it mean the difficulty is appropriate for me or that I picked one that's too difficult and would be better served by something that I can play with 1-2 hours of work?