A broody should stay on the nest all day and night, only getting off briefly once or twice a day to eat, drink and poo. If you bother her she will usually puff up and hiss and peck at you and crouch really low and flat over the eggs, a lower ranking broody may get bossed off the eggs by a higher ranking bird, but she will usually pace and fret near the nest until she can get back on, or go sit in another nest. Some birds will kind of practice being broody for awhile, maybe spend a lot of time in the nest boxes during the day and go back to the roost at night for a few days until they commit. But...some birds are never good broodies, that is they never get past a certain point or are too unsteady and will just leave or break every time any little ting bothers them.
One thing, when hens are just laying eggs, they will sometimes stay on the nest for an hour or more just to lay that egg, that is they don't just go to a nest, lay an egg in 30 seconds then leave. Younger birds especially will sometimes take a long time to lay, or birds that are having problems or have a really big egg to lay etc... so spending a couple of hours on the nest during the day usually doesn't mean much. Birds just laying an egg will often also peck at you and be annoyed if you bother them etc.
There are four or five good broody hen articles in this section of the Learning Center you might want to check out
https://www.backyardchickens.com/a/hatching-eggs-and-raising-chicks
If you have a committed broody, you can often move them and the nest into a separate cage if you can't separate her where she is, do it at night and quietly and good broody will still sit
Many people prefer to collect the real eggs and leave the fake ones in the nest until they have a broody they are sure is going to stick and is brooding where they want them, then give them the real eggs.
