Animisha Perhaps you are right. I read "lesson" as meaning a student-teacher meeting which (more assumptions) might be once a week or so. If the student is being moved on to a new piece every week that will limit the perfection that can be realistically expected, no matter how diligently the student practices. But if lesson is read as meaning a method book piece with the author's accompanying comments the student can of course spend all time needed to polish the piece further. But that might require significantly more than a year for completion of the entire book. Then again, I don't know how long each book was supposed to take.
Reading the preface to book 4 there is also an explicit assumption that the student will play supplementary material on the side, according to individual aims and taste. This will naturally cut into the practice time.
Personally I'm more inclined to believe that the intention was for the student to be exposed to and through sincere but not necessarily very extended practice incorporate the new style, musical concept, technical requirements etc into their experience and awareness so that it could be further developed the next time it was encountered. That might be doable for some students with one piece a week (while the undeniably fast progression in difficulty would likely make many others give up and drop out or - if they have a sensible teacher - a change in methods or more time per piece and book).
I'm also thinking that I'm guilty of having a perfectionist streak and likely read to much into the instruction to "pay particular attention" to this or that. As an adult and having played other instruments for decades I notice problems that I absolutely wouldn't have if I'd studied this as a child and as a first instrument. On the other hand, learning new motor skills seems more difficult with age so the gap between the goal in my mind and my actual execution is much larger making it difficult to give myself a pass to move on to the next piece. I'm trying to be a bit more lenient on myself.
I actually believe there are pedagogical merits to pushing a not yet advanced student along as soon as they have worked hard on a new concept and clearly grasped it even if there are still much to be improved on. Extreme control takes years to develop. As pointed out by @Sophia it's also a good practice to go back and review old material after some time when the general level has developed more. The more I think about it, the more likely it seems that JT had something like this in mind, not aiming for getting each piece to a polished level as for an exam or concert.
I bought books 4 and 5 earlier this year and have been working intermittently on book four and also played a little here and there in both books. But they've also collected a lot of dust while I've been working on other things. I'd say I've probably worked through the first third or so of book four. If I would choose to work on this series in a more focused way it seems to suit me best to study about three pieces interleaved for a month than to do one every week.
