So I've already have my primary character at 90 days from the date it was created on Brynhildr back when it was preferred (it lost its preferred server status on December), and I still get the buff as of today.

According to this link it was stated that:
Starting with the release of Patch 4.15, when the status of a World changes from New/Preferred to Standard, characters created when the World is New/Preferred will continue to receive their respective bonuses until 90 days after their creation.
I would be happy if it lasts longer since I still have several classes that needs leveling to 60, though that leaves me to wonder when it would actually expire.