for me.. might do around 3 hours per night or 4 hours when sacs come around. I don't know if that's enough though.. :\
4 hours is overkill. As EPL said get a routine happening of about 2 to 3 hours. This way you will have time to relax and do otherthings but also maintain full concentration and attention.
I know plenty of people that did a lot of study and it ended up backfiring since they had no time to relax and ended up being full of stress that they needed to do more and more.
If you do 2 hours of spesh a night you will hate on the subject and grow tired of it by the end of the year. I can understand doing 2 hours say Monday, Wednesday and Friday and having the Tuesday and Thursday just dont dedicate all your time to one subject and neglect the others.
Also the night before the SAC try to do no study at all for it and if you make it a maximum of 15-30 minutes to just check something you have forgotton. Otherwise you will have a mind full of information when it likely already knows it and you are overloading and making it do too much when it doesn't have to.
The brain is a simple tool that works well when not over used, people just try and do too much and wonder why why get stressed out.