I had one of my broodies get off the nest for 3 hours and still hatched her eggs later. She did it one day, the rest of the time it was the normal off for 20 to 30 minutes then back on them. It was also summer, so the temps were warmer. -I have a broody SS who has been sitting on the nest for 4 days-- her first time. Problem is, the last 2 days she's gotten off around noon for 2+ hours! Both days I thought she must not be going through with it, but then she's back on the nest not moving. I've read that some chickens leave for longer periods of time than the typical 5 or so minutes, but I don't know if those chickens always leave for longer times or just occasionally stay out for hours. She stays on all night, stays on through the other chickens being mad at her hogging the nest box they like and climbing in on top of her to lay, and she fluffs up her feathers and garbles at me when I get close, so those all seem like signs of sticking. She has water and food right outside her nest, but she still wants to leave and eat grass in the yard every day.
So far she's not on fertile eggs, but I have some and was planning to start her "officially" on Monday. I'm worried she won't be able to maintain the warmth if she keeps taking her frolic vacations. I moved her nest box tonight to an isolated place away from the rest of the flock, so we'll see if that helps. Other thoughts? Is this likely to work?
Good luck and some how Broodies know what they are doing. We can only guess and hope....