Users may have already noticed alerts for a new feature rolling out in Google Hangouts, which adds a timestamp that indicates the last time a user was online as part of the contact's status (e.g. 'last seen 2h ago'). This feature is available on Hangouts in M+Google Mail (gradual rollout), as well as on the latest versions of the Hangouts Android and iOS apps. You can control whether this info is shared from Hangout chat settings.
For more information, see "Last seen" indication in Hangouts