I am only answering your post because no one else has jumped in. Your canna is cold hardy to zone 7, which I think is the zone you live in, so you should be able to leave the plants outside.
At some point you will probably want to put them in the ground if for no other reason than because if you don't the pots will burst. Your plant is supposed to get full sun to partial shade. As far as dividing I think someone from Southern U.S. will have to comment. Up here where it is a little colder I pull my cannas in the fall and divide them in the spring. If the plants don't need to be pulled I'm not sure what the best procedure would be for dividing, or when to divide. Looks like a great plant- good luck.

Hi quicksilver
Thanks for your help. I will plant them in the ground.