Buy the 2015 Old Farmer's Almanac!
Often, one can make the finalOften, one can make the final harvest after the plant has died back, unless you desire a smaller sized potato, in which case the main harvest would be earlier, when most tubers have reached the desired size. If you do not have frost in your area, the plant still has plenty of green leaves (along with the new ones), and the plant is not starting to die back (turning brown), then you can certainly let the plant continue to grow and see if it survives to form more tubers.

2015 Garden Calendar2015 Weather Watcher's Calendar2015 Recipes Calendar2015 Engagement Calendar 2015 Everyday Calendar2015 Country CalendarNew Year Cross StitchLobster Rope Doormats