I would assume, that when the server isn't listed here anymore, as either new/designated or high population aka congested, that this buff won't apply anymore. But that's purely based on my assumption and experiences I have with SE and the earlier game/expansions. I doubt that they would let you keep the buff, after the population has settled, the whole reason of implementing these bonuses in first place. Might wanna keep up with the upcoming live letters and see if there's some more information released on it, or with the news section in general, but to be save I'll recommend, leveling as quick as possible, just in case the buff is removed/vanishes. Then again there's already so much exp that has been implemented prior to SB in comparison as we all started ARR and HW, that even without the buff, leveling should be your smallest problem.