I think my LO was about 5 months old before I got my first one. I've had 3 since she's been born, which are all over the place, I have no idea when they're coming, but thanks to another post, I might be able to tell now by when I get seriously moody for no reason.
With my first I never came on between having him and falling pregnant with my daughter. But after her, I think it was a few weeks before I had one.