The answer to 'when to replace soffit' is that you should replace your home's soffit when it shows signs of significant wear, damage, or deterioration. Soffit is the material that covers the underside of your roof's overhanging eaves, and it plays an important role in protecting your home from the elements and maintaining proper ventilation.
Common signs that it's time to replace your soffit include cracking, warping, or sagging of the material, as well as visible signs of water damage or rot. If your soffit is allowing pests or moisture to enter your home, that's also a clear indication that it needs to be replaced. Most experts recommend replacing soffit every 15-20 years, or sooner if it's showing its age.