Well I'd imagine once they earn an income if you can afford to keep it up until then, I have to say I kept paying mine until nearly 30! A bit much I know but made life a bit easier for them and I could afford it. I wouldn't be inclined to cut them off in college as presume they will have to let it drop altogether without an income to pay it.
Then on the other hand many people don't have health insurance at all, it's all about the level of risk a person is prepared to take.