Infection Rate of 'Auto-pilot' Sites

Has anyone come across any studies/reports/etc that put a number on the amount of known infected web sites that are using so-called "auto-pilot" scripts that make it possible for web site owners to just "set it and forget it" ?

It seems like the worst thing you can do with a web site is not check on it for months at a time. I would think checking on a web site any less than at least once a week would be too little. I'm guessing these auto-pilot type sites would be prime targets for hijackers.