According to the patent, which was filed in September 2011, the tab assassin would automatically shut down tabs that fall below a certain tab usage threshold. That threshold can be defined via "a period of time or an amount of activity". If a predefined or user-activity adjusted threshold is met, the tab assassin would automatically remove the tab. However, the patent also covers a "stored tab repository [that] may store information about the automatically removed tabs so that the removed tabs can be restored."
The activity level may be rather difficult to determine, since some tabs may simply be running in the background as part of a standard workspace. Google's idea to figure out whether a tab is significant or not includes the "viewing time of a tab", and comparing the "viewing time of each tab with a tab viewing threshold time and pass the tabs that have times that do not exceed the threshold to [the] tab assassin or removal." The system would also allow the user to set a period of time over which a tab would not be killed.