Hi,
      you must study atleast 8 hours a day . Theres no need to study all the subjects in a day but do revise the subjects. Make a time table if you wish so that your time is efficiently utilized.

Hi Nandish,

Now, having been a science student myself, I'll tell you this much, you HAVE to stay in touch with the subject throughout the time. The moment you develop a little laid back atitude, things will start piling up and you will have a tough time. 

Now, coming to how much time you should spend, that totally depends on your capacity. Some people can sit for almost 10 hours a day and some can barely sit for an hour or two. It's not about how much time you spend studying, Its actually about how you study, your methodology, your approach to the subject. 

See, visiting Physics and Chemistry twice or thrice every week will be more than enough but when it comes to mathematics and biology, you need to work these two subjects far more frequently (everyday). 

Also, when studying at first, mark all the important points and dervations and if there are any formulas in the topic then, make a list of it and maintain it. It'll help before the exams.