You might never get a broody hen. You might suddenly have six or seven of them.
Even if they are a breed known for going broody, yours might not. Or you might find one of your "non-setting" breed girls has decided she wants to be a mommy, and goes broody. It's an individual thing with each hen or pullet.
As another poster said, if you get one, never get rid of her!
I have a slew of different breeds in my flock, and my BO has gone broody once so far, as well as a Welsummer. They were barely 7 months old at the time, only been laying about a month. They are both done with raising their chicks, and haven't shown any sign of being broody again. Yet. I hope that's a "yet!"