It's not common to replace the spring at all. Basicallly, only if the spring breaks or the car is squatting. The stock spring is only one size, but there are different size pads that sit between the spring and the upper mount.

__________________
Mike Heath
1988 560SL Black/Palomino
1988 300SEL Black Pearl/Burgandy
1984 500SEC Anthracite Grey/Palomino