Depends on the roads you drive, the size of tires and wheels on the car and how you drive it.
I've seen all original parts on 150K fox bodies in good shape, and I've seen 75K parts on 99-04 Mustangs worn out.
I replaced struts, and inner and outers on my 50K 96 Cobra. One strut was junk, the other was fine. The inners were junk, but the outers were fine. The ball joints were in good condition, so I left them alone. Got the alignment and the car drove much better, even though it wasn't that bad before.