I've already fast-traveled a couple of times, but it hasn't worked. Do you think it's just a matter of time?
After a period of time, your follower will walk back to their home base. That will take some time too. So, yes, it will be a matter of time.
Unless she wandered off into the void.
You could load the save you have from just before you went in that dungeon. You do have that save, right?