ASA site-to-site IPSec vpn to linux ipsec-tools endpoints stops working after a random period of time
The cause of the issue was that the version of racoon in CentOS (ipsec-tools-0.6.5) appeared to have a bug with regards to properly re-keying. I compiled the latest ipsec-tools from source and the problem has not recurred as a result.
TL;DR - Upgrade ipsec-tools first before banging your head repeatedly on the wall.