Under good conditions there's no reason a corn should not be able to shed it's skin in a 24 hour period. How the shedding process works is when that snake is ready to lose that skin it will excrete water from it's body between the new skin and the shed to be. This makes the new shed easy to maneuver the shedding off. If the snake is too dehydrated, it will not be able to excrete enough water to make a successful shed.
Retaining shed can be a serious problem and should be taken care of with immediate action. Failure to do so could cause infection of areas on the snake. Most usual place of retained shed will be the eyecaps and tail. After your snake it is important to check the newly shed for a complete shed paying high attention to the tail and eycaps.
After the water has dry completely the retained shed will become tight and restrict blood flow like a tourniquet, and thus cause the infection and or death to the snake.
To prevent this, if your snake has not fully shed within 24 hrs of starting the actual shedding of the shedding process, grab a container with sealable lid and make air holes so it can breath and either put a wet moist paper towel or a wash clothe (some people like the wash clothes for their added texture, but i just use paper towels) Leave it be for about 45 mins - 1 hr. If the snake has not remove the rest of the retained shed, hold the snake and let it slither through your hand snugly, the shed should come off by now. In a more extreme case that i've seen from rescue-es, that have been left for days or weeks with the shed, you may have to rub gently to get that retains shed off.
Hope this helps.
And remember to keep your snake well hydrate to reduce the risk of this. It's not a 100% measure of prevention because some snake individuals DO have problems shedding, it will greatly reduce the risk.