how long do files hang around in temporary://?
Temporary files that are created with the managed file functions are deleted when they are older than six hours, and during cron tasks.

The description for the FILE_STATUS_PERMANENT reports the following text:


  Temporary files older than DRUPAL_MAXIMUM_TEMP_FILE_AGE will be removed during cron runs, but permanent files will not be removed during the file garbage collection process.


The value of DRUPAL_MAXIMUM_TEMP_FILE_AGE is 21600 (seconds), which means six hours.