My understanding is it's a bit of a bind and release process that moves the water from outside of the muscle cell to inside of it until the cell is saturated or the creatine is consumed.

So the amount of creatine changes the rate as which water is loaded into the cell, but the total amount of water needed is calculated based on your muscle mass. The figures I've seen is 2L extra water (spread evenly, not all at once) for a normally recommended 5g daily creatine intake during loading (which typically takes a month at 5g/day dosage).