Count only successful installations, where the user effectively uses Linux at least as often as he/she uses Windows. Dual-boot is allowed.
And yes, I'm a zealot
Also you can tell us more info, like which distro you used, and whether or not the user was computer-savvy.
I converted 4 people, all to Slackware, two are savvy, two aren't (so I do all the admin for them).