Here's a report on my experience checking in JFK:

I got restless @ my hotel lobby and decided to head to airport anyways around 6:30 PM. There was a line for Frankfurt people... I headed to the "Premium Economy" line which I believe the FRA flight doesn't even have this class of service. The person directing the line immediately asked me where I was going and when I said SIN she said I needed to wait until 3 hours before flight or for all Frankfurt customers to get taken care of. Finally around 7:30 PM the economy line died down to nothing. I again tried to check-in and the agent said I needed to wait for 8 PM. There was several other Singapore-bound passengers and they all started flooding the ticket agents at this point and finally they started checking us in. So I did get my boarding pass shortly after 7:30 PM.

They are pretty strict about the "Frankfurt passengers first" rule so I wouldn't try getting to airport overly early otherwise you'll just be waiting landside JFK which isn't so fun.