I get mine done when I do my DMV inspections...once a year.
Based on current mileage, that works out to just under 10K miles per change.
2009 ML350 (84K) - Family vehicle
2001 CLK430 Cabriolet (71K) - Wife's car
2005 BMW 645CI (124K) - My daily driver
2012 Mustang V6 (60K) - Daughter's car