I had it several times before (I go around kicking myself

out of my hood sometimes),and the notification did go away. It is the same kind of system notification such as (not in exact wording) "SOMEONE has opted you out of derby" or "SOMEONE invited you to join the neighborhood...".
These notifications is personal so only the concerned player (you in this case) can see it on your farm. If you have a second farm joining the same hood, the second farm won't see this notification.
I'm saying 30 days from memory but to be sure I'd say 40 days max. Would be nice if someone can share a similar experience.
Edit: silly me, just found this thread from 2014; the notification migth stay there up to 60 days. Sorry I misinformed you
https://forum.supercell.com/showthre...er-about-inbox