Andy,
The circumference of a circle is pi (approx 3.14) multiplied by its diameter. You could therefore approximate the length of a coil using:
circumference = 3.14 * diameter * number of coils
where the "diameter" is measured to the centre of the "thickness" of the coil if you see what I mean. (I'm assuming that each coil rests on the outside of the next inner one).
Hope this helps,
Paul.