The last couple of years we had an early start on beans up here. At unifoliate stage we got frost ( 28 F ) for about 2.5 hrs. This was the absolute borderline as some patches had the leaves burned and killed them while others survived and shot out two stems where the unifoliate used to be. We did not replant, but any colder or longer and the whole field would have been killed.

Keep an eye on them. By late afternoon or evening you can see a dark shimmer over the field if you got hit by frost.