Rohan Goel (CS Undergraduate at BITS Pilani KK Birla Goa Campus Says): 
Must Check : CBSE previous year’s Solved Board Papers 
I can tell what I did:
Was a student of Delhi Public School Ghaziabad. Never went to school after class 10 except for the some days which mostly include saturday when others chose not to come.
Used to religiously study at home and was preparing for JEE.
Never studied for what was taught in school/CBSE.
Attendance in Class XI and XII : <33%
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Percentage in Class XI : 79%
Percentage in Class XII Midsems : 78.2 % [6 Months to Boards]
Uptill here i haven’t opened the CBSE text books. All these marks are from what i learn at the coaching for JEE.
Uptill now my focus was only on JEE
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Pre Boards – 1 (Opens NCERT for the first time; just see important formulae ) ; Percentage : 82% [3 months to Boards]
Pre Boards 2 : ( Took school exams seriously for the first time and read all the chapters that too first time) ; Percentage : 86% [ 1.5 Months to Boards]
From 2 months before I started to give 1 hr a day to CBSE Prep.
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1 month to Boards:
Gave up JEE Preperation, Full focus on Boards.
Final Boards Percentage : 96.2%
JEE Advanced Rank : 5K
JEE Mains Rank : 2K
BITSAT Score :362
That too because physics got screwed. 🙁 Scored 90 in physics. Can predict the marks in rest. And mind you I had Informatics Practices and not Physical Education as the optional.
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So in a nutshell you can score >95% even if you start preparing just 2 months before but do that seriously.
You can as of now focus on JEE if that is your aim, which is more important.
Boards can be dealt and won at a later stage too.
Hope it helps.
All the Best.
Edit : You may choose to start practicing English from 3 months before if you aren’t good at it.