For every violent crime committed using a gun in the U.S. about 4 crimes are thwarted by people with a gun.
From a DOJ report that can be seen at http://bjs.gov/content/pub/ascii/hvfsdaft.txt :
“On average in 1987–92 about 83,000 crime victims per year used a firearm to defend themselves or their property.”
83,000 per year is more than 9 times per hour that get reported. A survey determined that for each occurrence that gets reported, there are about 23 occurrences that go unreported. The total crimes thwarted are then about 217.9 per hour.
That was the last report on this issue that the DOJ prepared before further reporting activity was cancelled by then President W. J. Clinton.