Try 'running this up the flagpole':
Use an Initial Interval to start off with, then based on the results of the following inspections and testing/repair data; decrease interval by half for a Stuck-type failure [hard failure], decrease interval by 1/4 for 'soft' failures like chatter-requiring-lapping, failure-to-reseat, etc. Increase the interval by 1/4 to 1/2 the previous interval for each 'Test Only-no repairs required' result.
Clean process 3-years initial, 10-years max
Dirty process 1-year initial, 10-years max [includes steam]
Very Clean service* 5-year initial, 10-year max.
PRV protected by rupture disk 5-years initial, 10-years max
Rupture disks-Clean service 5-year interval, 5-year max
R/D's - Dirty service, 1-year initial, 5-year max
*nitrogen, dryed compressed air, hydrogen, chillers, methane/nat. gas, propane, etc.
Excellent & dilligent recordkeeping is a MUST for this program to actually work.