EllisLab/CodeIgniter — pull request #1479 by srtfisher (2012-06-14 18:33:08+00:00)
Title: A TTL of 0 will keep the cache persistant.

In the APC cache, a TTL of 0 will keep the cache as persistant. For the file cache, it should be the same. If you pass a TTL as 0, it will be persistant (and never expire).