Ususally a check is mailed out to you after a few weeks. If you like, call Koodo and see if they can send it to you sooner. As long as the account is completely cancelled, There might not be a problem. Never hurts to ask.
If there is a credit on your account you'd have to wait for 1 full billing cycle in order to get the refund just in case there was any overage on your final billing. Once the billing cycle has gone by Koodo can send out a cheque through the mail which can take 4-6 weeks.
I was under the impression that you would have to call in after 1 full billing cycle (to make sure that there were no overages (as Rocky said), but in order for the refund to be sent out that you would have to call in.