Struts and shocks are an item that doesn't necessarily have to be replaced at any particular interval. The wear and tear on a strut assembly is dependent on the type of vehicle you own and how/where it is driven. Many strut assemblies can last 5-10 years without any required repair or replacement given the right driving conditions. However, some car experts suggest replacing them at 100,000 anyways. In my experience this is not the case. A Mazda Protege I once owned needed the shocks replaced at less than 80,000 miles however the components on my Honda Civic lasted much longer.
With significant variability based on driving conditions and vehicle make/model, struts and/or shocks should probably be inspected at every oil change. Look for wear on the strut itself or on the rubber components. Rust and corrosion can reduce performance of the suspension system. Also look for leaning or sagging in your vehicle and perform a bounce test to see if the struts respond properly. Push down as hard as you can on a corner of your vehicle. Once you let go if the car bounces a lot it may be time to replace the shocks and struts. If there is no (or limited) bounce, then your shocks and struts may be performing correctly.