How long does it take to get hydrated once you drink enough water? And how much water should one drink if dehydrated?

A few minutes. Water alone is not the best way to hydrate when dehydrated. A sports drink is actually better as the sugar and electrolytes in it promote hydration at the cellular level. If one is participating in sports such as running or cycling, its best to consume a high quality sports drink such can be found at hammer nutrition, but gatorade works fine. In terms of how much to drink, your body can usually regulate water intake very well. A general guideline is to consume about 500ml of a sports drink for 1 hour of cycling for example.