When was Google Earth last updated?
Google Earth imagery updates usually happen about every 2-3 weeks but the updates are not to the entire globe but to cities and regional areas often in multiple countries. Google provides a KML feed to show where the updates are located and also has a notification service to receive alerts when your selected areas have been updated. See related question for details.
Google Earth Software Updates
Google Earth software is usually updated several times a year with a major update (e.g. 6.2 -> 7.0) about once per year.