As needed...
The building of the shocks, especially the valving and internal hydraulic flow, has gotten waaay better than it was 30 years ago..
Check for leaks, especially any stains around the shaft area, check for any signs of impact, dents in the body of the strut/shock, check for deterioration of the upper rebound/limit bushing(usually hiding under the collar).. Then do the classic bounce test... Get the corner of the car in a up and down sequence(I usually kneel on the bumper) and let off, car should stop bouncing in less than two cycles..
Road test for any abnormal noise and stiffness...
Whenever you feel like it....
I have a 92 Honda Accord with 260K miles on it. I know the front struts went away around 25K miles ago(been a Honda/Acura tech for awhile) but the design of the strut/suspension is where I can still safely drive the car, just not as comfortable on rebound dampening. The right rear strut recently failed, as it is making considerable compression noise. So depends......