Basically, for most operations under FAA Part 121 you will find that 180 minutes is the baseline limit. An operator can go to 207 and 240 minutes in very specific operations.
207 minutes can be done for parts of Norther Pacific operations.
240 minutes can be in certain areas below the equator.
Beyond 240 minutes can be done under even more specific circumstances, such as West Coast of United States to Australia, New Zealand, Polynesia, and other geographically relevant locations. 240+ minutes also comes with a bunch of new operational limitations and currency requirements.
If UA does have the 180 minute rule (which I am sure they do), then in some situations they are allowed to extend past it.
http://www.ecfr.gov/cgi-bin/text-idx?SID=dc4a5c12b89bc362ee3d8c509c03a537&node=14:3.0.1.1.7.30.3.2.22&rgn=div9