In his book, How to Cook Everything Vegetarian, Mark Bittman explains some of the science behind wilting greens. According to him, crisp fresh greens go all limp because of salt in the salad dressing. The salt draws moisture out of the leaves, causing the plant tissue to weaken and the leaf to lose its crispness.
How fast this happens depends on the amount of salt in the dressing, the freshness of your greens, and the type of greens themselves. But all greens will eventually wilt!
The solution? Don't dress your salad until right before it will be served and eaten. Literally, at the last second! Unfortunately, there's no bringing a salad back once the greens have wilted.