The snake will usually go out during the night and drink some water, or when you aren't around. I'm not sure how much they drink, but I believe they drink it 1-2 times daily as I have seen my BP drink from her water dish.

When they drink from the water dish, they put the front of their head in (where the opening of the front of the mouth is) and suck in the water. You will notice their jaw going back and forth too so they can drink. Since snakes' tongues are split like this "Y", they cannot scoop up water like dogs and cats. Their tongues are too skinny and is only used to smelling the area around the snake.