My '08 FJ with the Off Road pkg has 47000 miles on it. So far the shocks/struts all seem to be holding fine. I spend about 90% of the time on-road and about 10% on gravel/dirt roads sometimes hitting bumps or holes at around30-40 mph. What is the life expectancy on these shocks? Should I go by the same recommendations as the base shocks, or should the Bilsteins last longer? I just need to know when to start a suspension fund.