Broody hens don't usually leave the nest for very long at all. When my first hen went broody, I thought she'd been eaten by something because I didn't see her at all for about two weeks. Even though the nest was under my lawn tractor cart in the corner of the garden where I was all the time, I never saw her off the nest. Then I needed to use the cart for something and found the nest, but even after that, I never saw her off the nest unless I disturbed her. I'm sure she must have gotten off for a drink and some food now and then, but it certainly wasn't often or for long. They aren't all quite that dedicated, but still, I would think the eggs would probably be alright in the winter. You might have a hard time keeping the chicks alive once they hatch though, unless you have a place you can put them and their mamma and keep a lamp on them. They aren't going to do well in the cold. In fact, that's probably why chances of one of your hens going broody in the winter is pretty slim. Even breeds that are notoriously broody usually wait till spring or summer when the weather is more friendly to little tykes.