In my opinion, never! I realise that there are different varieties of bamboo and some are more invasive than others, but I had such trouble with a well established clump taking over and spreading, and being impossible to dig up that I would advise never to let the darned stuff out of the pot.

You can do it now if you can be sure to keep it well watered during the current drought conditions in the South. Beattie's right though - depending which variety it is, you may want to use a rhizome root barrier inserted around the planting hole.

And the root barrier needs to be at least 50 cm deep--more is better--and a very tough material, such as concrete, stainless steel, or 2 mm thick polyethylene.