Honestly, to prepare thoroughly and properly, complete 2 years are required, if you wish to score well and get a good college. Start studying in 11th itself!
You can prepare for other entrances simultaneously (If JEE, AIIMS, JIPMER or KVPY or SAT is what you mean). Many students crack JEE and score well in NEET and other exams too since the syllabus is essentially same (except the maths and bio part 😛 ) But chem and physics are pretty much same.
Best of luck!