Every year around fifteen thousand students appear for the NATA entrance exam, indicating the level of competition.
When you start your preparation early, you will get ample time for understanding the concepts in-depth, practice previous year's question papers for NATA, and revise all the topics at least once before the exam. General Aptitude and drawing are the most critical sections in the exam, which must be given maximum priority during your preparation.
Each one of us will have a different learning speed and grasping concepts. Few aspirants will take 1-2 hours to understand a single topic, while a few take just 25-30 minutes. So, it all depends on each individual. Since to be eligible for the National Aptitude Test in Architecture, you must score a minimum of 50% marks in class 12, which means you can not neglect your board examination preparation. Therefore, I suggest you to start preparing for the exam side by side your 12th standard to avoid rush after your board exams.
Again it is upto you.
All the best!
