how long should it take the average person to study all 14 ATPL manuals and be exam ready?
It depends entirely on your individual circumstances. Prior qualifications, experience, familiarity with any of the subjects being dealt with, type and schedule of your current job, general stability in your personal affairs (family, health, finances), etc., etc. One thing I do not personally believe counts for much (within reason) is intellectual ability.
All the stuff is high-school level, no maths a 14 year old could not do, and as for the physics, if you are vaguely familiar with the idea of the Earth being round and have heard of something called gravity (what goes up comes down, bit like the stock market) you should be allright.
They
can be done in four months and without excessive sweating while holding a full time job but that doesn't mean that
you should plan on getting it over with within four months unless you honestly think that's a realistic possibility in your own current situation, and/or you have nothing to lose by trying
Those exams are a pain in the arse, but take your time if you feel you need it. Life's not
that short.