I usually replace them and the shocks all at once every 100,000 miles or so.
You can do the bounce test. Push down hard on the corner of the car. If it bounces more than 2 times (up) they are in need of replacement. As far as the ball joints and tie ends go might as well do them also while your down there.
They will last over 100K but at least they are done for another 100K. Take a look at the bushings also once you get past 150K they start needing replaced.
I know it is expensive but over 100K miles the cost of replacing everything isn't that bad. To have a dealer replace everything with parts (you) supply.
Struts, shocks, tie end, ball joints, bushings front and rear, $2000, Local garage $1000 maybe. 1 to 2 cents a mile.