how much minimum time i have to give in order to score good marks in UGAT exam.

Once you know the syllabus, you should devote all your time for practice. If you really want to crack the exam and get a decent college, then you must study at least 8 hours a day. Your first goal should be complete the entire syllabus and make short notes for the revision. Once you're done with your syllabus, you can clear your doubts (if any). You can also prepare from the last year papers.