Weird question and not a black and white answer but:

I installed koni orange shocks in back in 2011.

so four years and about 15,000 miles later I am under the car putting in new control arms and sway bar and the shocks look like crap (orange paint is gone and looks pretty rusted out). Shocks seem to be ok in the "bounce test" but I'm replacing them anyway.

I assume the surface rust is normal and should not be an indication of piston condition, but how often should they be changed out?

 

You're right on the it's not black and white. Really depends on road conditions and the spring rates; this is why we see aftermarket springs blowing OEM shocks/struts prematurely. I am a little disappointed in the finish of the Koni oranges, saw right away that it wouldn't take long for the paint to start flaking. But the ride has been phenomenal, and as long as they aren't leaking or blown, I could care less what they look like.

 

Very hard to say how often replacement is needed. For street driving they should easily last a couple years before they start deteriorating enough to notice. Don't Koni's have a lifetime warranty?

My oranges have rust too but seem to be working fine. I think they are 2011 era (I bought them "lightly used"). I plan to put in a set of Yellows in the spring though, as I'm out-driving the oranges at the track now. They've done a good job up to this point though.

 

I would say 75k miles easy

 

Yes. For normal street driving, they should last a couple of years. I can say that 50-70k is good.