Duck Hen off her nest

[JRF0114]

Today I noticed that one of my duck hens that's been setting on some eggs was away from her nest an unusual amount of time. I went and looked at the nest and found that a snake had gotten into the nest with the eggs. None of the eggs had been eaten or broken, however, I don't know how long the hen had been off the nest. The air temperature this morning was in the high 50's, but the eggs were surrounded by straw and feathers (and, unfortunately, a snake.) I removed the snake, and while doing so, the duck hen returned to her nest with the eggs, and resumed her task of incubating her clutch.

UNDER THE CONDITIONS MENTIONED ABOVE, DOES ANYONE KNOW HOW LONG A SETTING DUCK HEN CAN REMAIN OFF HER NEST BEFORE HER EGGS BECOME RUINED?