What happens if a guy leaves a cock-ring on his penis for too long (thus getting an erection which doesn't go down for hours)? I think I remember hearing something about that it will damage the penis.

You can get leakage of blood into other tissues of the penis. This will manifest itself as 'bruises' which is not too serious but you can cause permanent damage if repeated too often. It can also be the cause of blood clots forming and that is potentially fatal.

LOL