How long can a duck leave her nest without the eggs going bad?

I've got a runner duck sitting on a huge nest of 21 eggs (she had help from 2 rouens) and for 12 days has set on them with only a 20-30 minute break now and then. However last night, before 10 pm the other ducks were still on the newly thawed pond and she decided to join them. She didn't return for 3 hours. Are the eggs history? They're in a well made nest of straw and feathers, in a box in the garage. The outside temps last night were still in the high 50s (f). I'm having a hard time finding this kind of specific info on line. I would like to stop worrying one way or the other because I suspect she'll do this again tonight. This is her first attempt at brooding, she and the others are about 11 months old.