Three years ago I kept track of my Speckled Sussex hen's broody intervals because I was planning on letting her sit on and hatch some Brahma eggs, and I wanted to know when to start saving the fertilized Brahma eggs for her to sit on. She has been the most prolific broody of my entire flock as well as very predictable since she was so clockwork in her broody spells.
I calculated she went between fifteen and eighteen days between going broody. At the time I didn't count her eggs since I often didn't know which were hers.
But that might give you a general idea of when you might expect your hen to go broody if she does so on a very regular basis like my hen did.