The best time to start the preparation for JEE Main examination is typically after the completion of class 10th or at the beginning of class 11th. This allows students to focus on the fundamentals of mathematics, physics, and chemistry, and build a strong foundation in these subjects.

Starting early also gives students enough time to go through the entire syllabus, practice and revise important topics, and familiarize themselves with the exam pattern and question types. Additionally, students can also take mock tests, solve previous year question papers, and attend coaching classes or online tutorials if they wish.

However, it is also important to note that everyone's preparation timeline is different and depends on their individual strengths, weaknesses, and goals. Some students may start preparing earlier, while others may start preparing later. The most important thing is to start preparing early enough to give yourself enough time to cover the entire syllabus and develop a good understanding of the subjects.