Broody Hens

I'm new to chickens, we got our current girls as hatchlings last spring.
I've had a few girls go broody over winter and two who are broody now. We have no rooster and I take the eggs away. So they are currently sitting on golf balls.
My question is, is there a certain length of time they go broody for? I feel bad for them and am thinking about buying some hatching eggs for them. But I would hate it if they got off the nest before they had a chance to hatch.

They generally sit for 3 weeks. I would try to access some fertilized eggs. It's really fun to watch the mom and her chicks.

I'm not sure how long they have already been sitting for though. I have 4 black australorps and can't tell them apart. So I'm just worried if I gave them eggs they might get off partly through incubation and the poor babies would die

You'll need to decide if you want her to hatch out some chicks, and how you will 'manage' it.
Do you have or can you get some fertile eggs?
Do you have the space needed? She may need to be separated by wire from the rest of the flock.
Do you have a plan on what to do with the inevitable males? Rehome, butcher, keep in separate 'bachelor pad'?
If you decide to let her hatch out some fertile eggs, this is a great thread for reference and to ask questions.
It a long one but just start reading the first few pages, then browse thru some more at random.
https://www.backyardchickens.com/t/496101/broody-hen-thread

If you don't want her to hatch out chicks, best to break her promptly.
My experience went like this: After her setting for 3 days and nights in the nest, I put her in a wire dog crate with smaller wire on the bottom but no bedding, set up on a couple of 4x4's right in the coop and I would feed her some crumble a couple times a day.

I let her out a couple times a day and she would go out into the run, drop a huge turd, race around running, take a vigorous dust bath then head back to the nest... at which point I put her back in the crate. Each time her outings would lengthen a bit, eating, drinking and scratching more and on the 3rd afternoon she stayed out of the nest and went to roost that evening...event over, back to normal tho she didn't lay for another week or two.
Water nipple bottle added after pic was taken.