There is no any restriction for the gap between 10th & 12th to apply for AIIMS Entrance exam. As per the eligibility you should be passed Class 12 from CBSE/ISC/State Board with Physics, Chemistry, Biology and English as the main subject for General and OBC aspirants – 60% aggregate. You can check the further eligibility with the given below link.

https://medicine.careers360.com/articles/aiims-mbbs-eligibility-criteria

Good Luck!