I would suggest you take a mock exam and identify your weak spots. Spend solid 2-3 days to study the chapters that are giving you nightmares. Once that is done again take a mock test and see how you fare. If you think you have improved in the weak areas then move on and revise the other portions.
If you feel there is still some problem with your weak spots spend no more than a day to go through those chapters again. Once you are done with it revise the other portions of the syllabus.
Solve as much sample test papers as you can because level of questions in BITSAT is not much high and all you have to concentrate is mainly on speed and accuracy .