This is a really, really, really frequently asked question here at Flagpole Gear.  

The answer nobody loves:  It depends.

While the average lifespan of a good quality flag flown from sunrise to sunset is anywhere from 3-5 months, lots of factors can affect the actual length of time between replacements.

1. Wind and sun.  These are the major culprits for reducing the life of any flag.  Obviously, this will vary wildly depending on where you live and your weather any given season or year.  We often have people ask why they buy the same flag time and time again, but it won't last the same amount of time.  It's an easy answer - for most of the US, winter and summer don't have the same weather.  Dealing with different weather and temperature elements cause different wear.  Also, living in the city, where smog and exhaust fumes are more prevalent, can cause colors to dim faster, so even folks in the same area can have different wear.
2. Flying time.  Your flag can last nearly 3 times as long when only flown from sunrise to sunset.  The theory for this is that the flag fibers benefit from rest.  Remember that if you do chose to keep your flag flying 24/7, United States flags are to be properly illuminated as part of the US Flag Code.
3. Material.  Only use flags created for outdoor use.  These flags are made out of more durable materials, as well as having been dyed or treated to make them more fade resistant.  There is also a difference in the quality of material - not all polyesters, nylons, and cottons are made the same.  See our post on flag materials here.
4. Carelessness.  Your flag needs to be checked often.  Period.  Just because you decide to run it up the pole and fly it 24/7 doesn't mean it's self-maintaining until you see it's in tatters.  Inspecting it frequently allows you to check for loose threads or mini tears at the fly end, which is the first place most flags show wear.  Once you see these tiny flaws, trim the loose threads and re-hem the fly end.  This is a huge extender on the life of your flag.

 So what can you do to make your investment last a little longer?

1. Check your flag.  We mentioned this in #4 above, and it bears mentioning again because it is the absolute best thing you can do to keep your flag in good condition.  Trim and re-hem the fly end when you see the tiniest bit of wear (don't wait for an actual tear!).
2. Choosing the correct size of flag.  Here in Bristol, we have a grocery store chain that loves having a huge US flag in their parking lot.  We get it - everybody loves a ginormous flag!  The flag is nearly half the size of the roughly 30' flagpole, which means 2 things.   One, this thing almost never ever flies.  Shy of a tornado, wind speed high enough to fly a flag of that weight so low to the ground is rare.  Two, the flag is hanging limp against a dirty pole in a parking lot 24/7.  When it rains, it's heavier, and doesn't have the movement to fully dry.  So it's all folded on itself, day after day, allowing mildew to set in.  That takes all the fun out of a big flag, as well as their budget, because it needs to be replaced VERY frequently, and in a high visibility place like a grocery store parking lot, they hear about it if it isn't.  General rule is 1/4-1/3 of total above ground length.  This changes when flying multiple flags.  Check with your flagpole manufacturer on what your flagpole is meant to safely hold.
3. Purchase 2 flags to rotate.  This has actually been proven to extend the life of your flags - a theory is that the flag fibers benefit from having rest time.
4. Cleaning your flag.  This takes care, as doing it wrong can lead to disaster, or causing more wear.  This should be done only occasionally, hand washing with a warm mild detergent prevents dirt and pollutants from breaking down the fabric.  Make sure that the flag does not sit in the water for long, otherwise the red can bleed into the white.  Line dry.  Alternatively, there are also fabric sprays available that can repel dirt.