I always replace the primary spring twice a year and the secondary spring once a year but like mountainhorse said it is a little excessive but its added piece of mind since I run my sleds hard.
I normally replace mine every season or sooner if I suddenly have clutching issues...Have been known to put on 7000 miles in 1 season but that was a few years ago..but 4-5000 is common...
I have had springs last a decade and some last a season. The trick is to monitor your RPM and make sure you don't have a worn belt. Springs won't make the sled un rideable. Most issues are with a worn belt. Put a small sliver of bright tape on your tach helps monitor small changes.
I service both primary and secondary clutches a minimum of once per year (pre season) sometimes twice if I have been doing alot of WFO riding and put on alot of miles. When I service my clutches I replace pretty much all the wear parts (weight bushing's, springs, delrin washers, button's etc.).
In this economy $40 is $40. I remove and disassemble both clutches first thing at the end of the season. And pretty much wait until I'm ready to ride before I put everything back together. I rarely see 1000 miles in a season anymore so mine tend to last a few seasons.
Wifes 07 RMK 600 wasn't holding rpm at end of last season(would not pull full rpm)thought it might be the belt but maybe spring instead?Sled is right at 1000 miles. How do I know if belt is too worn,guess I could just swap out the belt to a new one,but there's no snow for a test ride. Pete