A year between each on the way up. I'm 37.
Now that I've got my B, though, my training has dropped off quite a bit (other stuff in life needs my attention), and it looks as though I won't be able to boost it to an A before the end of this calendar.

Standard response: It depends on your natural ability, the skills of your coach, fencing training partners, the resources (time/effort) you're willing to put into it, and attitude. There's no standad progression scale.