6 months is plenty for ATPLs. Don't worry about the time frame. Keep in mind it's just high school level stuff .. Anyone age 15-16 will not struggle grasping the concepts; They're often overly simplified anyway. It is just a lot of stuff to go through, and most of it will be irrelevant for your career so it gets tedious quickly. Just set a decent study plan, and stick with it, like a subject every 2 weeks?. Don't bother cramming, it won't work (~7500 pages in total?). It's all about sitting down with the books and taking your time reading through them. I made notes as I went along, noting all the important points and memory items like formulas. I ended up with a 250 page PDF document containing everything, I mean everything, you need to know and understand for all the exams. A lot easier to study than 7500 page books! Anyway, just do what works for you.
I don't see you getting a high mark by studying the material properly; Yes, the ATPL questions are that stupid. Study the books, understand the stuff, and then spend 2 or 3 weeks with the databases to ensure you'll pass. You can study your books from cover to cover and still have plenty of "wtf" moments when sitting the ATPLs.
Ask anyone if they'd pass the ATPLs again if they sat them RIGHT NOW. You would fail some subjects even if you sat them 2 weeks ago. Airline captains that could teach you the entire ATPL syllabus would not pass the ATPLs. The questions are stupid, and you don't get a lot of them (ie, you don't need to make a lot of mistakes on a 44 question exam to get in the low 80s or even fail).