I was staying at the Grand Hyatt near the airport, and they advised if flying in J class, that I could leave the hotel 90 minutes before the scheduled departure, since there wouldn't be much traffic at the time the CX flight leaves (ie 3.50am)
That sounds about right. Though you have to go though several lines ( front door, bagage screening, check in, imigration, security, and finalllly boarding), it's all quick in my experince (unlike DEL). Door to door you should be able to make it to you seat in 30 minutes max.
Don't foget a copy of your intenary. You will need this to get into the airport. Also, don't budget a lot of time for the lounge. They are all dumps at BOM.