Back when I was in H.S. I would leave them in for weeks; eventually the disposables will begin to disintegrate while at the same time accumulate protein deposits. I suppose it happens to some sooner than others, but they can adhere to your corneas.
Removing them is a painful process (like removing your tongue from a frozen flagpole), resulting in moderate to severe scarring on the cornea. Luckily, the vascularity in the eyes is such that it can heal quickly, but it will hurt to keep your eyes open for a couple days and it will feel like you have cotton balls stuffed in your eyelids.