Dear aspirant, I think preparing through self studies is a very subjective thing and it may differ from person to person depending upon various factors like one's ability to grasp concepts, his or her dedication, the amount of hardwork one is doing etc. For competitive exams like neet, aiims,jipmer etc you will require guidance which may be in any form be it your parents, school teachers or coaching. You should try to begin early and I guess 11th class is the point when you must start preparing seriously for these exams.

Hope it helps! 

If you are intending to prepare for the exams only through self-study, then remember that the earlier you start, the better. While the answer to this question varies depending upon an individual’s capabilities, it is generally seen that a majority of students begin their NEET, AIIMS and JIPMER preparations after entering Class XI itself. This allows them to spend ample time on familiarizing themselves with the vast syllabus of these exams, again based on Class XI and Class XII portions, and also lets them set aside sufficient time for revision towards the end of their Class XII, the period just before the medical entrances are held. Two years of time should be enough to focus on medical entrance preparations through self-study without having to sacrifice on the normal school exams.