Dear candidate, the time of completing the syllabus varies from person to person. Some students study the whole year till the syllabus is completed which is called regular study which is best. However, I have seen students who start studying after November and cover their whole syllabus in a span of three months and score above 90 percent but as I said it varies from person to person. Regular study is the safer side, why take the risk three months before the ISC exam. Start studying since the first day of class 12th. Make notes of important theorems, derivations, learn and understand terms and formulas by writing. Solve previous year question papers. Many questions are repeated in board exams. Buy 10 years book and start solving. ISC marking is all based on keywords. Focus on important terms in definitions and understand the marking, on what terms students get marks. Smart study is always better than studying everything. A small amount of hard work can easily fetch you good marks in the ISC exam.

All the best!

Hope, it helps!