When you purchase a harness, you should always check the warranty. More models will last for 3-4 years, but some companies offer a 5-year warranty for their products. However, this doesn't mean that you should just wait for the official expiration date to occur to toss your old harness and get a brand new one.
Experts recommend replacing a safety harness after a fall without fail, even if it's still within its warranty period. During a fall, all the straps that make the harness have to absorb around 1,000 pounds or more of force, which means that their structure is damaged, or at least weakened.
Therefore, the harness is compromised, and there is no way of telling when it will give in and fail to protect you. To make sure this doesn't happen to you, after such an event, make sure to replace your harness with a new one. Only this way, you will continue to benefit from the proper protection.