Sorry for the delayed response...your question is rather difficult to answer... defining a limit for one self can be a tough job..but following are my suggestions:
a). Follow the curiosity of your mind to decide on the limit:
Whenever you start studying anything (not just academics), your mind instantly asks questions about what it is, how it is possible, when does it happen and why at all this happens....once you have covered these four aspects of a problem or subject or topic, you become conceptually clear of the same.
To put an example in Innate immunity case only:
What is innate immunity? How is it established (which systems or components work to make it possible)? When (trigger points) does innate immunity comes into play? And why does one need innate immunity at all?
That's all one needs to study about in any topic or problem of life...if you miss on any one aspect, you will end up confused somewhere along the line in future.
(ii) Another way is to follow the minds of the subject experts. Now what do I mean by this ? Subject experts know what is important in a topic or chapter (remember in school days one used to ask the teachers to underline or tick mark important chapters for examinations?) This suggestion of mine works on similar lines but given the change in times, you might not be able to get someone do it for you. So, what one should ideally do is: Read each and every question/ fill in the blanks of the exercises given at the end of the chapters. Assess your self if you can answer them completely. Those exercises are meant to cover all important topics and sadly no one these days even takes a look at them.
(iii). For final few months before important exams, at any stage of life (school, college or profession), thorough following of previous year trends (questions) is extremely critical. Just go through the questions and if at any question (and its options) you feel little confused, just open your book or Google to clear your confusion. This practice essentially covers all important topics and also sheds the clouds of doubt and inexperience.
Hope it helps
MS(Research) Scholar, IIT Delhi (Alumnus)
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