The World Imagery basemap is not collectively updated. Both vector and imagery-based basemap content are updated approximately every three weeks with any new data that is provided by authoritative sources or the greater community.
The basemap is made up of several imagery tiles. In ArcMap, the Identify tool can be used to find out the date when a specific tile of interest was updated.
Additional information is available in the Related Information section, including metadata for the basemap and a list of all Community Maps Program contributors.