Do you mean that when you have reached grade 5, you have reached 1/5 of the level required to reach the ballades, or do you mean that you have 1/5 left, that you have reached 4/5 already?
I wrote "In terms of time...", I should have wrote, "In terms of time practicing..."; sorry for any confusion.
Usually as students increase in their grade, they practice more. Getting from grade 1-2 may only take like 20 hours (I'm throwing a random number out here) of total practice time while getting from grade 5-6 will take longer (remember I'm throwing a number to illustrate my point) along the lines of something like 50 hours. Keep in mind that everyone is different, but generally each subsequent level takes more hours of practice time.
Also, you asked about "being able to tackle Chopin's 1st ballade". My definition of being able to tackle something means being able to learn the notes and getting it near tempo in a reasonable time (for this piece I'd say no longer than 2 weeks of dedicated practice). Most people on these forums, by my definition, are not ready to tackle it but learn it anyway. The question is, are you willing to dedicate like 4+ months of a painfully slow study of it to just get the notes? If your teacher is willing to teach it to you, go for it, why not? I don't think it's the best growth piece, but with the proper guidance it can be very rewarding.
MUSIC ISN'T ABOUT FRACTIONS AND LEVELS! go practice. Aren't you the guy who wants to turn pro in the next 3 years?
This is so true! If you want to go pro in 3 years from grade 5, you should be practicing in your sleep (or even losing sleep to practice)! You shouldn't be worried how you compare to other people. I feel that if you can play a piece well, no one will care how long it took you to learn it.