Portal does not model Air Resistance (this is why you can alternate between two ground-based portals indefinitely, without losing height), though there does appear to be a terminal velocity.
But below that terminal velocity threshold, vertical distance remains the same -- fall 16 units, get shot up 16 units*.
*Assuming both portals are on the ground.
If you're mixing horizontal and vertical portals (or 45* angle portals), the distance will not be the same; as Jonathan Hobbs mentions in the comments, projectile motion comes into play, and you will go less high if your exit portal is at 45*, and less far if your exit portal is vertical (unless you also have room to fall downwards at the same time; hitting the ground cancels your momentum immediately).