There's no particular timings are given in their (AIIMS) undergraduate admission guidelines to fill the application form. You can fill your application form anytime of your convenience from the date of starting till the date of closing on their official admission portal ( www.aiimsexam.org ). 

Since, It is a national level entrance test, it is possible that their will be thousands of applicants filling their application form at the same time which can cause the admission portal to lag so I'll advise you to fill you application form at night.

Hello Dear
There is not any particular timing for that . You can be able to apply in any day under which the registration window will be open for taking the application form for applying in the AIIMS for the registration of the UG courses.
Thanks

All India Institute of Medical Sciences MBBS Entrance E ...

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