My AR is set for 50 yards (mistakenly hit 25 of the poll). With m193 and m855 ammo, it will should also hit close to zero at 200 meters. Along the path, the bullet should be within two inches from line of sight up to 250 meters or so. So, practically, a 50 yard zero means you can shoot point of aim/point of impact out to at least 200 yards if using an AR-15 for defensive purposes.
Notice I use the term "should" because I seldom shoot further than 100 yards with it. It is a bit high at 100 yds and a bit low at 25 yds, just as would be expected. See http://www.ar15.com/content/page.html?id=599
for more details.
Here's a nice chart showing the trajectory posted at http://www.m4carbine.net/showthread.php?t=63618: