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.