Tylan is supposed to be used for only 3-5 days, as it (from what I've heard) can damage the liver. Plus, it tends to work quickly, as it is a strong antibiotic. Of course, Tylan given orally doesn't work quite as rapidly as it does when injected.
At this point, I would weigh the prospect of your hen needing more days on Tylan to recover fully with the chance of continued Tylan hurting her liver or other organs. If it were me, I would probably give her the Tylan for a few more days and see what happens.
As for how long after her symptoms are gone to wait to re-introduce her into the flock, I would wait at least a week after the last sign of illness. Even after they recover, birds can still be shedding disease particles. Waiting a little while should not only reduce the possibility of her infecting other birds in the flock, but it will also give her time to regain strength before the stress of re-introduction.
Good luck with your hen! I, too, am treating a sick bird with oral Tylan right now. It seems to be helping, though she isn't out of the woods yet.