Very nice!
I have heard 10 years for the first foot or so, after that it speeds up.
I guess what they grow is indeed a caudex, but it really just looks like conical trunk here is a pretty big one at Boyce Thompson Arboretum (given that they grow them there they must be somewhat frost hardy).
I have also read that once they have leaved out you can water them a lot as long as the soil is fast draining and dries out pretty fast. In have a 6" tall one in the ground that just put some leaves on now that it is not 110+ any more and we had some humidity come through.
