As soon as your syllabus gets over (11+12), you can start taking mocks to judge your preparation level.
You can first try it casually- mocks given in magazines or some previous year papers of your institute. Get an idea where you lag behind ( if you fear a particular type of question, say mechanics, it means you need to work harder there.)
So if you start taking mocks as early as possible- you get an idea about your weak points.
I loved doing Biology mocks, especially from the Dinesh book of old question papers. The Kerala PMT’s had really smart questions ( in Biology) and I highly recommend them to all AIIMS aspirants.
Proper, timed mocks- you should give in the last one month or so. Helps you to build the strategy of how to attempt your paper ( the order of subjects) and the time they need to be given. For me, I followed-
- Physics ( not my strong point, attempted earlier when mind was fresh) – 70 minutes
- Chemistry- 50 minutes
- Biology( strongest)- 30 minutes
- Remaining 30 minutes- all questions that I wasn’t able to complete in my allotted time period/any questions I left due to doubts