The buff stays as long as a world is preferred, and a minimum of 90 days.

Which means, if 90 days expires from the time that you make a character on a preferred world, but the world is still preferred, you keep the buff until the world is no longer preferred, even on day 91+.

This also means that if you make a character on a preferred world, and its status is no longer preferred a day later, you keep the "road to x" buff another 89 days.

Hope this helps.