It is depend on the capability of an individual ,but in general 8hrs are enough to score well in boards.
I used to study 8 hours ,as I gave my 2.5 to 3 hrs to maths ,1to 1.5 to chemistry ,2 hrs to physics and rest to hrs to Eng and optional . From January to March these hrs are extended upto 10 hrs . And I scored 93 percent in my boards .
Good luck 👍👍, hoping best for you.