The Sylvanas fight ends when you get her health to 50% on Normal and Heroic modes. There has been speculation that you may require a lower health for completion on Mythic, supported by an aura datamined by Wowhead:
Sylvanas. The RWF leader, guild Echo, discovered that the speculation was correct when they won the race to get Sylvanas to 50%: on Mythic Sylvanas, the encounter ends when her health is at 45%. Echo, taken by surprise and not having accounted for this possibility, wiped at 49.6%.
