I had to change mine out at the end of my 3rd year of ownership. I think you just have to look them over per the following tip sheet and judge your own situation. Weight of fiver, roads traveled, luck of the draw will all factor in. Note that small cracks are normal and not an indication of a worn out shear spring.
http://www.morryde.com/uploads/downl...%20UPDATED.pdf
There are several full timers and long timers on this site that are regular repliers. Their RV lifestyle and valued additions such as the I/S independent suspension upgrade may not be the answer to all members on this site. The LRE 4000 rubber suspension on like mine and probably yours work well for our needs ... we are recreational campers only spending 6-8 weeks a year in our unit.