How often or at what milage should the struts and shocks be replaced on 2004 Chevrolet Impala

Our mechicanic said our struts and shocks are OK (milage 138000) We bought Goodyear tires at Sears ( NEW ), with about 50000 miles on them, and now they are starting to cup and Sears said our struts and shock should have been replaced at 50000 miles Is this true

Asked by for the 2004 Chevrolet Impala
50k is the recommended time interval for struts.