There is no exact answer to that question. The life expectancy of a flag will depend on the elements that it is exposed to. Regardless of fabric type and construction features, flags sooner or later will succumb to the elements. Sun, rain, wind, ice, and snow all weaken flag fabrics and threads.
Many of our customers report that by taking their flag down during the windiest 5% of the days they double the life expectancy of their flag.
The U.S. Government generally expects a nylon flag to last approximately 90 days based on daily usage from sunrise to sunset but not during periods of inclement weather.