I got great advice on the same question related to shocks.

http://www.peachparts.com/shopforum/showthread.php?t=178008

I decided to change the four shocks. They were relatively cheap and the outcome could'nt be better.
Now I am wondering on the very same issue but focusing on the springs instead.
What do you think ?
Will springs outlast shocks or should they also be replaced based on a double entry table: time / mileage ?
Of course I am speaking of springs with no evident problems. The discussion is not about broken/not broken but about ride/comfort IF replaced.
Thanks, Jorge