Cookies will either time out on their own, OR your browser will clear them out (manually or automatically). Take Firefox, for example. In your menu bar, go to Tools> Options…> Privacy and look at the cookies area in this panel. You can clear your private data (including all cookies) by hand or set things to clear every time you close your browser. You can also view the cookies that have loaded onto your machine from here and you will see expiration information for each.
So—while companies may like to set their cookies on users’ machines for their own time periods—they are ultimately at the mercy of the user’s sophistication, preferences, and paranoia. This means the data collected from cookies will NEVER show the whole picture, though they may offer a decent snapshot at times.