I can't remember how long it took for mine to get that big. I think they can get up to twelve feet, so it's good to know that they can take a hard prune and bounce back. Maybe the height is also determined by the climate and soil. The bees sure do like them though and it's a host plant for the Silver-spotted Skipper butterfly among others which is why I have it. I haven't found any eggs on it yet though.
