[Steve]

Hello everyone, been a while since I checked in here, but I have not been on FB either because I despise FB.

How long SSPs were in service has no definite answer. It is entirely up to each agency's policy and where the vehicle was assigned and how it was used. SSPs assigned to patrol would likely have the shortest in-service time, but again that depends on the agency. If they were taken off patrol and used for another purpose they might have stayed much longer.
What I can tell you about is CHP and early SSPs. At the time, CHP would retire patrol-assigned vehicles at 70,000 miles to preserve resale value. My '82 was retired and sold in late summer of 1984, about a 2 or 2.5 year time on patrol. That was in an area where we covered a huge territory and I put about 200 miles a day on it in average use. In less expansive areas it would likely be less and they would stay longer.
Later on CHP did away with the 70k limit and I hear these days they run them until they fall apart. Frankly I would afraid to do high speed operations in a worn-out vehicle.
SSPs not assigned to patrol, and there were some used for Administrative purposes, could be around for many years.