Kitty's power seems to be terminated when the traveler has caused sufficient damage to the timeline to cause her to cease to exist. We see this most clearly when Professor X (at the bidding of Wolverine) manages to convince Mystique not to kill Trask. At the precise moment of decision, the timeline fractures and all of the Sentinels (and the X-Men in the room) disappear. Presumably the ruined Earth disappears as well as the timeline disintegrates.
To a lesser degree, we see this in the scene with Bishop. There's no indication that she's stopped doing whatever she's doing, only that she senses that the timeline is breaking and that there's time for a witty quip before she disappears.