Actually, yes some people do have 6 Prometheans. It takes a while but not a long time, about average if you play wizard 101 for 4 hours a day for about 2 months, you will reach Promethean. So 2 months times 6 is 12 months so it will probably take about one year. I'm think about it in terms of just doing the storyline quest and no side quests.
I know many people with more than one account having on all of them all max level wizards including me :D Some of them made twice like fire, myth and balance so yeah to your question is yes, there are many people with 6 and more max level wizards :D
I have 7 promethean wizards and another that is very close. (I run 2 accounts).
I also have several friends with at least 6 prometheans.
I play several hours a day, sometimes less, depending on my schedule. I normally don't start another wizard until I take the wizard I am working on completely through the game.