Depending on how powerful your character and the map it can range from 4 minutes to 60 minutes per map.
Most characters have only 5 maps, and only the main character has the most with 10 maps. With 1 map though, I can beat it in 19 seconds, it's a boss only map though. Usually though for first time runs, it's 30 minutes to 60 minutes.
Jolee: Ohh, HK-47... You're so hard and smooth! HK-47: Ohhh, Jolee... You're so soft and wrinkly!
Sephia campaign seems like the shortest, but I could be wrong. She only has a couple maps, and the last is just a short Boss fight. Her holy mountain quest ispretty much the longest and could take you anywhere from 40 mins to a little over and hour with out replays.
Galen seems to have the longest campaign. Some really long maps with extra hard bosses towards the end. The crystal caves took forever, where as the colonei canyons only took me 30 mins.
Magnni's seemed fairly long. But,you can always plow through a stage just by passing like 75 % of the enemies. That will seriously cut down on time. I didnt have any trouble with Maggni until the boss at Tramont defense, which was a royal pain in the ass. Magnni's final mission is a real challenge, its not that its long its just that you will prob have to repeat it many times.
Levv...I didnt really have the time to level him up and he starts out much too weak, so I wasnt interested in completing his campaign. by the time i got past crystal caves Ive had enough. He just dies too easily and theres too much delay in his attacks. But i think he only has a couple maps.
Zazi, well I found her campaign to be the easiest. I leveled her up really well and everything was a breeze. The second to last boss for her, wasnt nearly as tough as Behemoth, or Magnni's.