My experience is that a kid who likes music will PLAY for hours, but PRACTICE for hours...well, perhaps rare before age 11.
And there I think you've hit the proverbial nail ...
I do agree very much that some kind of guideline is useful, and perhaps even necessary, and that it's something that THE book could have usefully tackled (perhaps it did, and maybe those who've read a copy could tell us).
However the difficulty with this is that it's not just a question of putting in the time. Practice is just as likely to be the wrong sort as insufficient - it seems it's very often a combination of both.
A chart can suggest length of time, amount of practice, but the crunch is going to be contained in the quality of that practice time - whether it's proper practice or glorified messing around / playing the bits you already know. Half an hour of superficial playing through can very easily sound more productive than 15 minutes of thinking interspersed with bits of playing - and yet the latter may well be achieving more. 'If the practice sounds good it probably isn't real practice' may be a useful motto. Not an easy one though.
