Window screens should generally be replaced every 10 to 15 years, but this can vary based on several factors. The frequency of replacement depends on the material of the screen, the environmental conditions, and the level of maintenance. 
Aluminum screens, known for their durability, can last up to 20 years if well-maintained. Fiberglass screens, while more affordable, typically last around 10 to 15 years. In areas with harsh weather conditions, such as high humidity, strong winds, or extreme temperatures, screens may need to be replaced more frequently due to accelerated wear and tear.
Regular maintenance can extend the lifespan of window screens. This includes cleaning them periodically to remove dirt, dust, and debris, which can cause deterioration over time. Inspecting screens for damage, such as tears, holes, or loose frames, and repairing minor issues promptly can also help prolong their life.
If screens are exposed to pets or children, they may suffer from additional wear and require more frequent replacement. Pet-resistant screens are available and can be a worthwhile investment in such cases.
Ultimately, the decision to replace window screens should be based on their condition. If screens are visibly damaged, no longer fit properly, or fail to keep out insects and debris, it is time to consider replacement. Regular inspections, ideally once a year, can help determine when replacement is necessary.