front and rear struts

[Jchan]

how many miles do you start having problems with the suspension. when to replace them

[terrastrife]

you replace them when you have problems (or ideally at the end of their rated service life).

[73sport]

Two things:
1) Pull the strut boots up and look for oil residue around the shaft. If its leaking it will soon stop dampening.

2) Got to each corner of the car, give it a good solid push downward with all your weight. Compression dampening (down) and rebound dampening (up) should be the same on all four corners of the car.

If one corner bounces a couple of times and the others don't, its time for replacement.

Good Luck.
/randy