I'd expect the MINIMUM duration for any ban to be a week. I've banned people anywhere from a week, to a month, to a couple of months, to a few indefinite bans that are still in effect.
The best way for any ban to get lifted, in my opinion, is to demonstrate that you have learned your lesson. Which, in your case, would require not posting a string of pointless messages everywhere at the drop of a hat.
"May the Grace of Ara go with you, and His Vengeance be wrought upon your enemies."
best thing for u to do in my opinion is to find a forum the the banner moderates and type a public apology, but i am not an admin so i can't say weather it would work or not. Its different for each person i would think. It probably also includes how many times you did what you got banned for if you are a repeated offender you may just need to except the time that the ban was placed on you. but again i am no admin so their is no gearentee(sp) that this would work.