If your body is just slightly dehydrated then once you drink some fluids such as water or Gatorade or something else that is hydrating then you should begin getting hydrated again within 45 minutes to an hour.
It's important to drink fluids even before you feel thirsty to prevent dehydration in the first place because if you get severely dehydrated then it can lead to hospitalization and even death if not treated in time.
Severe dehydration requires IV's at the hospital to get your body back to the hydrated state it needs to be.
Severe dehydration needs emergency help so if you think yourself or someone else is severely dehydrated you need to call 911 or get the ambulance on the way or get to the hospital if possible.
Dehydration can lead to unresponsiveness and death if not treated.