There is a big variation of how long it takes. For some, its a matter of hours, while for others, it takes days. When its off, I don't know how long it takes, but I would imagine it takes longer time. Dell claims that turning off their LCD tv's doesn't fix it at all:
Turning the LCD monitor off for an extended period of time does not clear the image. The charge must be bled off by charging/discharging the pixels with random images.
link
From what I have read, most people who have had a sucessful recovery used the black and white solution at night while operating the computer normally in daytime. After a 2-3 days, it was gone.
I would recommend that method, since its the one thats reported most as successful.
Edit: Here is a link you can read describing the same method. Hopefully it will help you feel the effort is worth trying:
http://www.screen-wipes.com/pc-screen-burn-fix-trick-for-plasma-and.html