The answer to 'when to replace surge protector' is that you should replace your surge protector every 3-5 years, or sooner if it has been exposed to a major power surge. Surge protectors are designed to absorb and dissipate excess electrical energy to protect your valuable electronics, but their protective components wear out over time. After several years of use, the surge protector may no longer be able to provide the same level of protection.
It's also important to replace your surge protector if it has been subjected to a significant power surge, such as during a lightning storm or other electrical event. Even if the surge protector appears to be working normally, the internal components may have been damaged and the unit may no longer be able to fully protect your electronics.