By default, the CallTrackingMetrics tracking code will cookie website visitors for a 30 day period. This means that if a visitor comes to your website from an advertising source you are tracking and then they come back within that 30 day period from another source, the code will remember the original source they came from and will continue to show them the tracking number associated to that source. If they come again after that 30 day window, a new cookie will be placed with the source information from the new visit.
You can also choose to enable last touch on your advertising tracking sources in CTM. By turning on last touch, you are telling the system that if a visitor comes through this advertising tracking source, it should override any previous sources the visitor came through.