If the "S" is composed of two simple curves with the radii known, then it is very simple:
Radius * ArcInRadians
What information do you have about the wall.
Most architectural "S" walls I have seen are designed using spirals. So a bunch of little technical details are needed to calculate the "length".
Is the "length" defined as the reference point on the front face, or some point in the middle of a cross section, and is the wall completely symetrical about the inflection point?
Do you have more data?