I normally hate waiting at the airport so I always go at the last minute, resulting being I did miss a few flights here and there. However in India I must suggest you to go as early as 3 hours - add an hour more if travelling during peak season like Christmas. Reasons being:
There are a LOT of people! A lot of them travel with many checkin luggages, for some reason I found my 3 flights in India all had very slow checkin services, I had to wait for almost 40 min even during midnight flight just to check in
The security is strict and very slow too. First you are required to present your flight ticket and photo ID to security guards for getting into the airport. Normally there are only 1 or 2 of them checking so takes forever when the line is long, and they do their job well by looking very closely. The security check after checkin is also very slow, ladies have to go through a separate screening plus there are double windows open for men than for ladies! ?(If you are a girl travelling alone, make sure you keep as many things possible into your bags and lock them in case you are asked to go to a far away screening window and will have to leave your things lying in trays waiting)
They board the flight super early. Im used to boarding gate opening 30min before the flight but in India they seem to start as early as 1hr before, and their last call is around 30min before the flight. Apparently for domestic flights if they can board 90% of the plane they are allowed to take off early so they often rush you.