See depending on your foundation the time parameters will vary. ISI/CMI both entrances are conceptual. So you have to be conceptual as well as you need to have genuine problem skills as well as attachment to the subject. So if you think you have weak foundations then start preparing from 2nd year of your bachelor course. If you think you are in a good foundational state start in final year. But it is always advised that you should start preparing as soon as possible so that you can practice more which results in time management as well as increases your ability of problem solving.

To know more about CMI you may visit the link provided below:

https://www.careers360.com/university/chennai-mathematical-institute-chennai

I hope my answer helps. All the very best for your future endeavors!