If we meet at/leave from the Triangle (LIE service road @ Little Neck Parkway) it's about 105 miles to the Montauk train station. Call it 7 hours at 15mph. If we go to the Montauk Point Lighthouse add another 12 miles (60-90 minutes). I'd say if we started rolling at or a little before 7am, we could hit Montauk by 2pm, including 3 quick stops along the way, if we make better time than that, we can go to the point and come back and still make the 3:30 train back to Jamaica.
The 3:30 is supposed to arrive at Jamaica by 6:10. Riding from Jamaica station back to the Triangle (assuming you parked there) is about 9 junk miles. So, assuming everything works out you'd be back in your car by ~7pm.
If for some reason we missed the 3:30, there's a 5:35 and a 7:33.
It's definitely a huge time sink, no matter how you slice it.