There are 2 kinds of broody hens that I know of. The ones that are actually incubating fertile eggs and the ones whose hormones tell them that they are supposed to park their butts in a nest until the shavings or fake eggs or golf balls hatch. Being I know you are an intelligent person, I know that you know that won't ever work.
Now I've never had a hen that was incubating eggs but as I understand how it works:
- The hen lays eggs for a period of time, could be 6, a dozen maybe more
- She decides it is time to incubate said eggs and stops laying.
- She will park on the eggs for a few days straight
- After they start developing she will get off the nest for a period of time each day (maybe 1/2 hour) to eat, drink, poop.
- The last 2-3 days before they hatch (21 days) she'll park her butt in the nest.
- Eggs hatch, little chicks hang out under mama's wings.
- Mama teaches them everything they need to know, protects them, calls them to food. I don't know if there is a general timeframe that this lasts but when I gave Zorra the 2015 chicks she was a Tasmanian Devil (the Warner Brothers type) if any of the other hens (with 2 exceptions) came within 5 feet of her babies. She's a big girl to start with but "grew" to twice her size when "necessary". 2 months later she laid an egg and forgot all about the chicks, even started chasing them away from food.

Zorra and her week old babies in the brooder area of the coop.
BTW, this is a great example of the lie that claims young chicks need an ambient temp of 95°F the first week, 90° the next, then 85° then 80°. The barn was never anywhere near 80°. 45°to 55° at night, 55° to 70° during the day. What they NEED is access to a "warming hut" when they get cold. Hens of course are the ultimate warming hut since they move to wherever the chicks are exploring. The next best thing is a Mama Heating Pad cave. The chicks learn that is where the warm is and will go back under whenever necessary. The least desirable (yes I AM biased) is a heat lamp in a small space where they never get away from the oppressive heat and 24 hour light. They cheep constantly day and night, cheeping is a complaint. Happy chicks chirp quietly and sleep at night. With a heat lamp they have no idea of a normal day/night cycle.
A broody hen (in my experience) that has nothing to hatch will:
- lays eggs for a period of time, could be 6, a dozen maybe more. We people steal the eggs daily.
- her hormones say "park on the nest and hatch whatever is there, even nothing.
- they will NOT get off the nest for a daily constitutional, likely will growl and puff up when anything comes near.
- if another hen lays in the nest (as with Angel in the 4' long community box) they will gather those eggs under them with whatever they already have ... or don't.
- they will fuss when you reach under. I gather some can be quite vicious though I've been lucky.
- If you take them out of the nest they will be kind of Mr. Hyde like. Running around all fluffed up, squawking their broody squawk. But once out they will eat, drink, poop. Broody poop, being backed up for a long time, is BIG and STINKY! Then they will go back to their nest.
- They will sit in that nest for weeks and have the potential to do harm from lack of food and water.
- Unless one has fertile eggs or fairly newly hatched chicks to give them, they should be broken.
Breaking a broody hen entails getting them elevated in a wire enclosure of some sort, I made one with 1/2" hardware cloth on all sides, top and bottom. No nest material. The idea is to keep them from staying warm underneath which breaks the hormone cycle. My buster box sits about 3' off the ground.
Obviously it is easier to break them when the temperature is colder than in the heat of summer. How long it takes to break them depends a lot on how long they have been broody. If you catch them the first day and it is relatively cool, they might break in a day or two. Let them stay broody because you (wrongly) think it is cruel to break them and it will take a LOT longer. Generally they won't lay until 5+ days after they have been broken. Some people claim that you should dunk their breast in icy water and boom they will be broken. I don't think so. I once had a girl that had been parked for a couple of days. She thawed two refreezable ice packs in succession that I put under her and when I took the second one out (thawed) she was hot as a furnace underneath.