The key will be autogenerated only after all the necessary fields are set. Which fields are necessary to generate the key depends on the format that is used for the cite key. This format can be set in the Cite Key Preferences.
Of course the key will never be automatically generated if you do not choose the autogenerate option in the Cite Key Preferences.
When a key is automatically generated, a message will appear in the status bar of the Editor window, if that window is open and the status bar is shown.