The cookie that Google adds to a user's computer when he or she views or clicks on an ad expires in 30 days. This measure, and the fact that Google uses separate servers for conversion tracking and search results, protects the user's privacy.
Users who don't wish to participate in tracking activities can easily not accept this cookie by setting their internet browser user preferences to block cookies. This may need to be done again after a user clears cookies in order to maintain these settings. These users will simply not be included in your conversion tracking statistics.