Chennai Consulate schedules an interview immediately after receiving the packet from the NVC, but they don't inform you.

They just send out Packet 4, but that might take over 2 weeks (like it did for us!). So I would suggest writing an email to the Consulate as soon as you know your case was sent there (with the NVC-given case number in the subject line, and in the body of the email including your name, beneficiary name and date of birth, and full address that you want Packet 4 sent to). We found out our interview date after emailing them. Chennai is great, they email back within hours!