This article discusses Router Lifetime value advertised in IPv6 Router Advertisement messages.
An IPv6 router sends periodic as well as solicited RA messages out its advertising interfaces to the all-nodes multicast address. RA messages inform about router's ability to forward IPv6 packets and contain other parameters required for Stateless Address Autoconfiguration.
RA message includes a value called Router Lifetime (in seconds). This value applies to the router's usefulness as a default router. It does not apply to other information contained in the RA message. IPv6 hosts receiving the RA message should install the default route with an expiry time set to the Lifetime. A Lifetime of 0 indicates that the router is not a default router anymore and associated default route should be discarded from host's routing table.