I am in 12^th std , how many hours of study

is needed for 1) board exam

2) IIT

each day

Scoring good marks in IITJEE and Board depends on various factors.It does not depend on how much hours you study.It depends on the combination of thing
a)proper planning
b)effective study
c)smart hard work
d)support
e)dedication
The numbers of hour depends on easily you can grasp the understanding of the topic and how effectively you put your concept in solving numericals.
So if your concept is not cleared in less time you need to put more effort and vice versa