Experience has proven that this is an impossible question to answer accurately. It is like predicting the weather, airborne contamination, and the treatment people will give a flag.
The major enemies of a flag are wind, water, sun, and carelessness (the single greatest cause of flag deterioration). Neither you nor we can control the weather, but you can take care of your flag and lengthen its life. Occasional washing in warm mild-detergent water will prevent dirt and pollutants from attacking the fabric. To prevent mildew, let your flag dry thoroughly before storing it. Have your flag repaired at the first sign of fraying, don't' wait for it to be blown to shreds. Continuous day and night display will shorten a flag's life. If your flag is not illuminated at night, you should consider taking it down to appreciably lengthen its life.
Our flags are manufactured to give maximum service in return for reasonable care and prudent use. Remember, no two flags receive identical wear. Because weather conditions vary, wearing conditions vary and consequently, the life of each flag is different.
Your flag works hard. It shakes... it trembles ... it drapes ... it whips... it snaps ... it chafes .. it bakes ... it freezes .. it ripples ... it flutters .. it quivers ... it furls ... it rolls ... it twists ... it flaps ... it strains ... it flies ... it unfurls ... it hangs! Is it any wonder that is needs to be replaced two or three times a year?
The best way we know to stretch your "flag dollar" is to have three flags: one flying, one in the wash, and a clean one in reserve for special occasions.