It takes them 21 days to hatch. They will keep their young chicks with them for a few wks before they start to wean them. By then they would do ok on their own, under normal circumstances.
Just seperate her from any nests or eggs. She will calm down. It only takes a few days actually. I usually keep them sep. For a wk just to make sure.
Some breeds are more broody than others. Bantams usually go broody more so than standard size breeds. And some bantam breeds are to the extreme. I.e. Silkies, cochins, seramas, old english games etc.
I occasionally have one of my sex link hens go broody. I immediately seperate her. Her job is to lay. Once they go broody it takes them a few wks to get their system back in order to start laying.
The reason why its not good for their health is both physical and mental. Broodiness is a mental thing. If you continue to allow them to think they are setting and they are not. It messes with their minds. Plus after 3 wks on a nest a hens body is pulled way down. Not just weight but the inside workings. So if they go beyond the natural 3 wk period. You can imagine the snow ball effect. Even if feed is carried to them or you throw them off the nest so they eat. Its not the same. A chicken naturally eats all day long. They are browsers of sorts. Except for the afternoon rest periiods of an hr to two hrs.
Sorry I've rambled. I enjoy discussing fowl. I've had chickens all my life.