Typically how long does geographic tongue last?

Varies. The activity of geographic tongue will wax and wane over time and may even have periods where there is no lesion activity. Some people are able to identify environmental stimuli that increase lesion activity although or is not common. It will continue to "move" with one part of the tongue resolving with another area becoming affected.
Reference Site. 1) http://en.Wikipedia.Org/wiki/geographic_tongue 2) http://www.Mayoclinic.Com/health/geographic-tongue/ds00819.