The majority of permanent bans are simply 30 day bans which are re-administered again before they expire each time. Admins can also extend any ban to any number of days they wish. For example a while back there was a user banned for a number of years.
The ban screen looks the same as a normal ban screen, obviously.
However, these days, most accounts are deleted rather than having a large extension to the ban time. Most permanent bans which are updated by moderators keep the accounts off the forums until they are reached in the deletion queue.