I've typically gotten 6-8 months per flag, although some have gone a full year. Usually by that point they are starting to fade and tatter pretty bad. The winter is tough on them due to the freezing and thawing, and the summer sun can bleach out the color over time.
It's something to think about if you want a flag if you need to buy one twice a year every year.
Depending on the size and quality of the flag it can be quite expensive over the year to fly a private flag.