Quote Originally Posted by Chrallmal View Post
The buff only works on classes/jobs below level 70. However, how long the buff lasts is not keyed off of the level of any class/job. As such, if you level a job to 70+, the buff will remain for other sub-70 jobs to enjoy until such time that the buff falls off of your character as a whole.

I've been told that the buff will last as long as the server is on the preferred worlds list. That could be days, weeks, or months. When I started my current character on Zalera, I had the buff for 500+ hours of playtime, and managed to get several jobs to 70 in that timeframe. When I created a character on Brynhildr, it only lasted about 200 hours. While I still managed to get several jobs to 70, I didn't have the time to do more before the buff wore off.
To be specific, the Preferred/New World bonus lasts either for 90 days or until the Preferred/New World's status changes to Standard/Congested, whichever comes later. So if your character is older than 90 days but the server has been Preferred/New that whole time, the bonus stays until that server's status changes. Conversely, if you create a new character on a Preferred/New World and it changes the next day to Standard, you'll still retain the buff for the next 89 days.