The answer to 'when to replace gutters' is that you should generally replace your gutters every 20-30 years, or sooner if they show signs of significant wear and tear. Gutters are an essential part of your home's drainage system, and over time they can become damaged, clogged, or detached from the roof, leading to water damage and other issues.
Some common signs that it's time to replace your gutters include cracking, peeling, or rusting of the material, sagging or pulling away from the roof, frequent clogs, and leaks or overflows during heavy rain. If you notice any of these problems, it's a good idea to have a professional roofing contractor inspect your gutters and recommend whether they need to be repaired or replaced.