Figure a mother plant will probably stay around 6 months or so before you 'retire' her for a smaller and less scruffy one. A 3-5 gallon pail will do fine, and you can read up on bonsai mums and root pruning under I believe advanced techniques, search it.
My advice for keeping moms is to use even LIGHTER soil than you normally would. Since the plant will live in the same pot for a long time, and you may be cycling among different fertilizers (bloom for the week before taking cuts, grow for the weeks after) you'll be flushing regularly, and lighter soil will make that a LOT easier.