I took a B6 domestic flight dp 0640 yesterday, and while the terminal is probably the busiest terminal at JFK at that time of day, 1.5 hours prior to your flight should be enough assuming you don't need special care or have many bags to check.

I regularly sail through B6's terminal--but I never check bags, travel light and arrive prepared (checked in) to pass through security with offending items removed (metals, liquids, etc.).

You should be fine.