In the end, it's only forever.
The clock struck twelve, and still Sarah slept on peacefully. To amuse himself and to pass the time, Jareth gently sat her so that he could inspect her hair. It was black, and brown—and blue and green and a violent color of red he found as his fingers wound their way through her thick locks, threading magic behind them. The magic brought out and enhanced all of the colors which the human eye could not see. Magic would bring out a lot more of Sarah than probably even he could predict. It would all show through, however in time.
And he would need time, enough time to keep her forever. Because humans didn't live forever, they weakened and grayed and died. That would not happen to his Sarah. His fingers itched for an occupation, so he started to braid her long, magical hair. At a quarter to the thirteenth hour, Jareth hit upon how he would save her for himself. Steal her from her human fate. He would take a slice of eternity, he decided, and craft a necklace, a ring, and every bauble she might ever want out of it and wrap her fully in them. They would keep her from aging, and so long as she continued to dream, only half awake, she would never notice. She would never grow bored with how long her life extended, and Jareth was content with that. Sarah content in dreams, and Jareth content with Sarah.
With an errant wave, the bubble the little child was contained in drifted towards him. There was a merry bounce to it that made him smile as he looked at the red and white clothed infant. In ten minutes, Sarah would have lost the baby to him. And then she was his as well. He had failed to mention that unless she succeeded, she would have to find her own way out of his Labyrinth. She probably assumed he would send her home as a reward for her victory.
But Sarah, he knew, dreamed of her victory, of her life, of her adventure. Tomorrow she would dream of being a princess, and the next she would dream of being an explorer, the day after a vivid adventure in the Outlands.
So he crooned a soft song, a lullaby, into her ear as the minutes ticked down to moments. A whimsical smile tinged his face and voice as Sarah's and Kiffle's time passed into eternity. They were his now, already gone from the minds of those Above who had known them. If Sarah ever asked who Kiffle had been, Jareth would tell her the truth—Kiffle was a wish-away, one who Sarah had fought very hard to keep. And if she ever asked where she had come from, he would tell her the truth too: She had walked into his life out of a dream. He'd leave out the part that she walked right into her own dreams, that he'd trapped her here very much on purpose. His smile widened as his song slowed, eventually reaching a sweet silence. The little prince had been selfish, but he had won.
Author's Notes: As I wrote this, it has become kind of my own personal canon for Labyrinth. Not exactly the happy children's tale, I know. But Jareth, Hoggle and I suppose the Wise Man are the only more than vaguely humanoids in the Labyrinth, all the rest are a bunch of crazy, mismatched characters. That's where I came up with the idea that Jareth was a wish-away. I severely dislike the Sidhe, Unseelie/Seelie, elves, wizards, Celtic magic, druid magic, fairy circles, etc etc that many stories resort to to explain Jareth. I feel like the answer to him is the very fact that his kingdom's population is based on the numbers of the unwanted. Eh, I'll leave your Jareth-backstory alone if you leave mine alone.
Also, Jareth is incredibly, incredibly vain and selfish. I thought of him as a wish-away, combined with that, and felt that the rest of this story wouldn't be out of line with the stuff he'd get up to. Not everyone gets a happy ending, either, but I think Jareth and Sarah each got their own happy endings this time. If only sort of.