It's not like if you study 3 hours a night you are guaranteed to get a 99+ ATAR
That said, the people at my school who got a 99+ last year studies, like 1-2 hours a day minimum.
http://community.boredofstudies.org/showthread.php/257754-Why-does-english-standard-and-advance-scale-the-same-according-to-ATAR-calculator
Haven't you asked this already?