From my friends' experience, February or March is the best time to attempt your GMAT for YLP at ISB Hyderabad. The logic behind this has been explained below.
The Round 1 is a basic filter round that evaluates you on the basis of your essay, extracurriculars, and academic credentials.
Now, Round 2 evaluates you on the basis of your GMAT score, essays, LOR, resume and attachments of other valid credentials.
The usual gap between Round 2 and 3 is 4 months. Usually the time given for GMAT preparation and Round 2 application submission is 4 months. But if your profile is lacking in any aspect, finishing the GMAT by February and March gives you additional time to improve your profile through essays or other categories. This increases your performance in the Interview.
So the best way is to complete your GMAT before Round 1 to avail maximum benefits.