The recommended interval for replacing shocks is between 50,000 miles to 100,000 miles.
If you drive on more rough roads then the shocks will get more wear and tear faster and need replaced sooner than if you mainly drive on roads that are smooth such as highways, freeways etc.
The more bouncing your vehicle does the more wear and tear your shocks will get and then you will need to replace them.
Some shocks may last the vehicles life as well and you usually don't need to replace the shocks unless your vehicle is bouncing a lot or you notice other ride comfort issues which the vehicle.
Most of my customers who come in have me replace the shocks after every 50,000 miles and that's okay with me.
Sometimes the shocks are just fine but they want to replace them as a preventative measure.