What time do you arrive JFK?
What time is the SQ flight?
Transfer on the free AirTrain will take about 8 minutes. Mid-day, trains run every 4 minutes. Take the train that loops around to other terminals. I believe thats the inside track.
SQ check-in when we flew business (Raffles, was around 10 minutes). To be on the safe side, allow 3 hours.
If I recall correctly, SQ uses one of the other * lounges at the terminal. It's small, with some hot food choices. Does SQ first allow use of the Virgin lounge?