This all depends on the dehydration state. Someone that is mildly dehydrated can rehydrate within 45 minutes to a hydrated state if he consumes enough water.
It is of extreme importance to note that people who are in a severe dehydration state should receive emergency medical attention immediately. Typical symptoms of severe dehydration include vomiting, muscle cramps, disorientation, or unconsciousness. This is not the time to try and rehydrate a person at home. It is a medical emergency and if not treated correctly it can be fatal.
