Most manufacturers of shock absorbers advise replacing them at 50,000 miles, but that's probably more for their benefit than yours. While shocks from decades ago may have been spent at that point, improvements in design and construction allow today's shocks to go much further under normal driving.
Shock absorbers, which dampen spring oscillators and give you a smoother ride in the process, typically last between 50,000 and 100,000 miles. Factors that affect the longevity of your shocks include bumpy roads, overloading, and stop-and-go driving.