It's very variable. I didn't have cycles until 19 months pp and then was very irregular until about 5 months ago. We casually TTC from 15 months for nearly a year with no luck, and have just conceived in the second cycle following a 6m break from TTC.
I saw so many stories of people getting pregnant while still breastfeeding tiny babies that I didn't think it would be a problem. I'm now pleased about the gap we'll have, but wish I'd known how long it would take at the outset!