SEA Tigger should probably chime in here. My understanding is that a plane goes through a significant overhall every 20000 hours of service which on the 73's could be every three or four years. The average life of 73's is around 20 years before metal fatigue forces it to the boneyard. If an airline has cash, the paint job needs to be done every five to seven years which takes the metal out of service for a week or two. Combine that with a 20000 hour overhall, the configuration could also be gutted at that time.
Sorry for a long guess, but let's say 7 or 8 years.
Again, SEA Tigger could probably give a better answer.