My, possibly percieved by some to be harsh, opinion.
If you need more than a week or two to learn a booklet of, what is it, 10 pages? Then you will not pass the courses.
You will, in particular for the first few weeks, be bombarded with information. I believe the pace is 7 lessons per day (average might be slightly less but expect at least 5 a day for an average of 6). You will be tested on it and although formally you don't have to, most people learn large chunks of manual verbatim.
ATC luckily is not that theory intensive compared to many other things (I used to study engineering and that was a lot worse in my opinion). However there are phases (beginning of a rating course and beginning of validation training) when you will have to absorb a large chunk of material in a very short space of time and apply it in practice. If you need weeks to learn a booklet, you're not going to like learning large chunks of an ATC manual verbatim. In particular not while juggling other subjets as well.
Might seem harsh but that's my take.
TL;DR
Two weeks is by far sufficent to learn the booklet, in all likelyhood one week would do but I realise people want to prepare meticulously so give it two weeks for a bit of margin.