I've found that a change the panel says will take up to two hours can take over five hours.
If your mailbox is a user mailbox (i.e. has a name you chose rather than m########), you can check the limit directly by enabling shell access if not enabled, and at the shell entering "quota". 1 block = 1Kbytes.
And be warned that when a mailbox is overquota one can get not only bounced messages but /lost/ messages - messages that the mailserver thinks it delivered to the mail box so does not bounce, but the mailbox never received because ISTM there is insufficient disc space for processing.