How long does she have to sit on the eggs before they hatch?
About 21 days. It can be 19 days or 23 days even with a broody hen, but usually pretty close to 21 days.
Will she do everything or would I need step in and pull the chicks when they are born?
Hens have been hatching chicks with the flock and raising them with the flock for thousands of years. There are risks with this. There are risks if you isolate them for hatching or raising them. You are dealing with living animals. There are some risks no matter what you do. Different ones of us have different set-ups, each chicken has its own personality, and each flock has its own dynamics. What works or causes problems for one of us might be totally different for others.
If you mark all the eggs you want her to hatch, put them all under her at the same time, and check under her daily to remove any other eggs that might show up, you should not have to interfere during hatch at all. I firmly believe the more you interfere, the greater chance you take of getting Mama into protective mode where she has a real chance of injuring a chick while preparing to fight you off. They are living animals and don't all have the same personalities or strength of instinct, but I believe you are better staying out of the way and let Mama handle it. Of course I watch to see if there is a reason to interfere, but I don't interfere because I lack self control or because it is neat or cool to interfere.
What I usually do is wait until Mama brings them off the nest. Then I put Mama and the babies in a prepared place for a couple of days so the chicks can learn to eat and drink without interference from the older chickens. Then I turn them loose to grow up with the flock. A lot of people don't go to that much trouble but I can be over protective. When you pick Mama up, be careful. Those chicks can crawl up under her wings. I've crushed a chick by picking Mama up, so get your hands underneath her so you can be sure you are not grabbing a chick along with her.
A good broody will defend her chicks from other hens, if the other hens pose a problem. Not all broodies are good, but not all hens are searching out the chicks, trying to steal them away from Mama for a snack. The other chickens may or may not kill the chicks if they get separated from Mama where she cannot protect them. My biggest problems with a broody raising chicks with a flock have been where the chicks get on the wrong side of a fence that Mama cannot get through to protect them. If you do decide to isolate Mama and her babies, make sure the chicks cannot get out of that enclosure without Mama going with them.
Will she ever get off the nest to eat and drink?
They are living animals with their own instincts and personalities. Almost all broodies will leave the nest to eat, drink, dust bathe, and poop. I find that if the weather is hot, they spend a lot more time off the nest than if the weather is cooler. And occasionally you will find a broody that does not get off the nest to take care of business. It does not hurt anything at all to remove her once a day, say when you are checking for new eggs, to give her a chance to take her daily constitutional, but don't be surprised if she runs back to the nest instead. And remember they are very protective of that nest. They are not likely to get off if there is a threat around. You being around is a threat. They are going to try to sneak on and off the nest.
I don't know what you mean about the nests being too high for chicks. I've seen broodies get chicks out of a 10' high hay loft. Mama says jump and they do. They are living animals and anything can happen, but I've never seen one get hurt doing this. I'm not recommending a nest ten feet high for fun, but I personally would not worry about a three or four foot high nesting box.
The other side of this is that a chick might fall out and not be able to get back in. I have decent lips on mine to keep the chickens from scratching out eggs and nesting material. I've never had a chick fall out. But it is possible, especially if the lip is pretty low.
I suggest you read my post on this thread. It might help or maybe help clarify some more questions. Good luck!
https://www.backyardchickens.com/forum/viewtopic.php?id=213218
You might crack some eggs and look for the bull's eye to see if they are fertile. A four month old may or may not be fertilizing the eggs.
Fertile Egg Photos
https://www.backyardchickens.com/forum/viewtopic.php?id=16008&p=6
How many to give her? I suggest a minimum of four to be pretty sure some will hatch. They can handle as many as they can cover. Not all eggs are the same size and not all chickens are the same size. Usually a hen can very comfortably cover a dozen eggs the same size she lays, but a bantam may have trouble covering four full sized eggs.
