The answer to 'when to replace asphalt shingles' is that you typically need to replace asphalt shingles every 15-20 years, or sooner if they show significant wear and tear. Asphalt shingles are a common roofing material, but they have a limited lifespan and will eventually need to be replaced to maintain the integrity and protection of your roof.
Signs that your asphalt shingles need to be replaced include curling, cracking, missing granules, and leaks in your roof. If you notice these issues, it's a good idea to have a professional roofing contractor inspect your roof and provide recommendations on whether a full replacement is needed.