I have, I've read most of the CDC metastudy that the top reply quotes. The entire quote:

Defensive use of guns by crime victims is a common occurrence, although the exact number remains disputed (Cook and Ludwig, 1996; Kleck, 2001a). Almost all national survey estimates indicate that defensive gun uses by victims are at least as common as offensive uses by criminals, with estimates of annual uses ranging from about 500,000 to more than 3 million (Kleck, 2001a), in the context of about 300,000 violent crimes involving firearms in 2008 (BJS, 2010). On the other hand, some scholars point to a radically lower estimate of only 108,000 annual defensive uses based on the National Crime Victimization Survey (Cook et al., 1997). The variation in these numbers remains a controversy in the field. The estimate of 3 million defensive uses per year is based on an extrapolation from a small number of responses taken from more than 19 national surveys. The former estimate of 108,000 is difficult to interpret because respondents were not asked specifically about defensive gun use.

If you ask gun owners directly about defensive use of guns, you can get numbers in the range of 500k to 3M defensive uses per year. If you get the data without directly asking, then the figures could be closer to 100k. If you ask people who pray how effective their prayer is, they're going to report it as being very effective. If you ask people who practice homeopathy the same question, they'll be able to a slew of medical ailments that homeopathy has helped treat. And if you ask gun owners, a group of people who have already decided that guns are an effective means of self-defense, how effective they are, they will exaggerate the actual effectiveness.

The 100k number seems believable. You take the 1.2M violent crimes. Assume 40% of those have victims who own guns, and then assume a very generous 20% of people who own guns just happen to have access to the gun and successfully use it in self defense, and you wind up with 100k actual defensive gun uses per year.