Both times I went, I arrived about 20 minutes before the sunset, and got situated. Once the actual sunset started happening, it was a matter of a minute or two before the sun was gone.
Just before sunset @ 24mm:
Sunset @ 70mm:
The peak that the sun sat behind is Mount Monadnock—about 50 km from Groton.
I took a couple of panorama shots. I like this one the best (6 shots):
While hiking up the hill, I spotted this tree against the colorful sky. I had to get a silhouette:
I was surprised at how little time the entire trip took. From leaving the house to getting back, it was about 70 minutes. This is certainly a quick photo shoot compared to the day-long trips like the one to Boston in early June ([1,2,3]).