From the website of a British flag manufacturer. (I've not provided a direct link as the site has an invalid security certificate):
"How long does a flag last?The lifespan of a flag depends on how it is made, where it is flown and in what weather conditions. This makes predicting the lifespan of a flag very difficult.Flags are perishable items, the colours will start to fade and the flag will begin to fray. Even the strongest of flags cannot be flown for long in strong winds without deteriorating. In general, a polyester flag will last approximately 90 days based on daily display, but not during periods of inclement weather. In some cases a polyester flag displayed 24 hours a day may only last a quarter of that time. Therefore a lifespan of 22 days is suggested.The weather is the main variable that determines the life of a flag. Your flag should be made up of an all weather fabric such as polyester. All hems should be turned back twice, secured with two rows of lock stitching on the fly end - if the flag is printed onto a standard polyester.All weather flags made of polyester will sustain damage in extreme weather conditions. Sun, rain, wind, ice and snow all weaken fabrics and threads. When your flag begins to show obvious signs of wear such as holes, severe fading or frayed edging it is appropriate to retire the old flag and replace it with a new one.We recommend that when you buy flags, you buy them in pairs as it is very difficult to predict the life expectancy of a flag due to climatic conditions and the number of hours you fly your flag from your flagpole. "
From a US website:
"A flag's lifespan is often in the hands of its owner. The U.S. Government estimates that a nylon flag will last about 90 days if it is only flown from sunrise to sunset in good weather. Flags flown 24 hours a day will typically last one half to one third as long. Larger flags have a shorter life than smaller ones because they are exposed to higher wind speeds and the fly ends have a stronger whiplash effect"
(Source:
https://www.united-states-flag.com/about-flag.html )