Local Scans take 4 hours, and may or may not reveal new information regarding enemy agents living in the same City as the CIA Office where it is performed.
Performing a Local ScanEdit
Each Local Scan takes exactly 4 hours to complete, whether it detected any information or not.
It is also sometimes possible for a Local Scan to pick up Clues about agents living in other Cities. This occurs when a Participant residing in the current city has already managed to conduct dealings with agents elsewhere. For instance, if the agent in this city has sent a message to another agent, there is a slim chance that a piece of Clue pertaining to that other agent may pop up during the scan. This does not necessarily require Max to know that the message has been sent.
Very often, a Local Scan will yield absolutely no information. This is especially true at higher Difficulty settings where information is harder to gather with any method.
Note that Surveillance Quality has no discernable effect on a Local Scan. Installing a large number of Wiretaps and Bugs around the city does not increase the chance of acquiring information through such a scan.
Local vs. International ScanEdit
Both these scans take 4 hours to complete, and to the untrained eye may seem to do the same exact thing - but there are indeed differences in how they work.
A Local Scan is more likely to pick up any information (as opposed to none). However, it can only work when the City you are currently in is the residence of at least one enemy agent, and will generally only pick up information related directly to that agent. It is useful when you know someone is hiding somewhere in a City, but don't know where specifically to look.
An International Scan can pick up information from all over the world, and so can reveal Clues regarding agents living in other cities. It does not require there being an enemy agent in the same city where the Scan is initiated. However, it is far less likely to generate any information at all. This method is more useful when you are absolutely stumped about your investigation, and want to find out which city to go to next.