I hope you have already covered your syllabus once. If you haven't, then do it.
If you have, then practice more and more sample papers. Don't just see questions and say i know that. Sit and properly solve the questions in the given time frame and after that, check the solutions if they are correct by yourself. It will be good if you give more importance to the long answer questions in all subjects except maths. If you do the long answer type questions, most of the short answers will be covered in that only. And for maths and physics, you need to give importance to all questions and practice more and more. It will be better if you could write the answers in your notebook instead of just reading. And regarding time, don't fix a time. Give as much time as you could. If you set a time, you can't concentrate properly. All you will be thinking of is the time that has passed.