How Long Does A Fly Live?

If I remember correctly, it's 3 days.

For the common house fly, three days would not necessarily be unusual in the warmer months where the entire egg, larvae, pupa, adult cycle can be only seven or eight days, but more typically, the adult phase will last btween two and four weeks.  Typical time spans for the various stages are
 
Egg  1/2 to 2 days
Larvae 4 to 10 days
Pupa 3 to 6 days
Adult 15 to 30 days
 
House flies mate about one to two days after they emerge from the pupa stage, and the female is usually laying eggs by the end of day two.  So even if they only make it to three days as adults, the war is still on.
 
Hope this is not more than you really wanted to know.

Unless it is in reach of my yorkie...3 days is correct.

It depends on how far it stays away from my plate.

I heard somewhere that they live about 1/2 month

Too darn long if you ask me. Three to five days I think.