An effect that has a conditional duration lasts until a specific event occurs. Three conditional durations appear more than any other in the game.
Until the Start/End of Someone’s Next Turn: The effect ends when a specific creature’s next turn starts or ends. The creature is usually the one who produced the effect or else the target of the effect.
Until the End of the Encounter: The effect ends at the end of the current encounter or after 5 minutes, whichever comes first.
Save Ends: The effect ends when the target makes a successful saving throw against it. See “Saving Throws".
Published in Player's Handbook, page(s) 278, Rules Compendium, page(s) 226.