There is no exact answer. The U.S. Government generally expects a nylon or cotton flag to last 90 days based on being flown daily from sunrise to sunset - but not during periods of inclement weather. Tests have shown that, in some cases, a flag flown 24 hours a day will last only one-fourth as long as one flown only during the daylight hours.
Regardless of how well it is constructed, a flag is, after all, only made of cloth and will sooner or later succumb to the elements. However, it has been well documented that reasonably good care can contribute greatly to longer life.
Note to Large Flag Users (12 feet x 18 feet and up)
Larger flags naturally represent a more significant investment, and as such, should be given maximum protection. The above advice of inspecting your flag bears repeating to keep your flag in good repair:
• Inspect your flag regularly for signs of wear. In particular, look for "normal wear" fabric or thread breaks which may occur in the "fly" end. This is the end farthest from the staff. Trimming off and re-hemming torn or frayed ends will help extend the life of your flag.
Consider having two flags and rotating them on a regular basis. Tests have shown that flag fibers actually benefit from periodic "rest". Also you will not be without a flag while one is being cleaned or repaired.