Yes, I'd give her 21 days. What kind of enclosure do you keep them in?
I have some in large-ish cages, and whereas they do go broody, they have left the nest in less than a week every time (3 times this far). But I also have two smallish aviaries with buttons and imo they seem to be even better at hatching their own chicks than chickens are.
I just got 9 in one of the aviaries yesterday, have 6 that are about 6 weeks old from the other aviary already. They almost breed like rats ^^ But not many people seem to share that experience.
Usually people say they don't go broody with any kind of regularity, are bad at incubating the eggs and sometimes even worse as parents.. Well, not so in my aviaries. I also have multiple hens in the aviaries and even the ones that didn't actually hatch the chicks, help raising them! Apparently the chicks can't tell mom and dad from aunt and when they hide under the other hens, their instincts just kick in and they let them stay.
Do keep an eye on them, though, should any chicks come - on one occasion I separated a hen for about a week due to her pecking at the toes of the chicks. She didn't want to hurt them, she just thought they looked like food and I separated her till the chicks got a little bigger so they wouldn't get hurt. And also, other people have experienced even the parents of the chicks attacking them, so as this is the first time, you don't know how yours will respond, so don't assume everything will be okay just because I say mine are great parents
If you are still unsure of the sexes of your birds, you can try posting some pictures. As long as the bird is not white in the vent area, it can usually be sexed by color. Males have a rust red - or, in dilute varieties like silver, pink-ish - hue to the feathers around their vent, which females don't have.
Make sure you have a proper feed for them. Grown birds can survive on poor diets like budgie seed for a while but chicks need something with much more protein. I use a game bird starter with 24.4% protein for adults and chicks alike and supplement the adults with ground up oyster shell as a calcium supplement for egg laying in a separate dish, so they can take as much as they need.