Drupal cache expire value of -1?
It is coming from includes/bootstrap.inc:

/**
 * Indicates that the item should be removed at the next general cache wipe.
 */
define('CACHE_TEMPORARY', -1);


Which is also defined in the docblock for cache_set($cid, $data, $bin, $expire):


  $expire: One of the following values: 
   - CACHE_PERMANENT: Indicates that the item should never be removed unless explicitly told to using cache_clear_all() with a cache ID.
   - CACHE_TEMPORARY: Indicates that the item should be removed at the next general cache wipe.
   - A Unix timestamp: Indicates that the item should be kept at least until the given time, after which it behaves like CACHE_TEMPORARY.