This directive is not available in the 3.3 version of Squid.
For older versions than 3.3 see the linked pages above
|Default Value:||shutdown_lifetime 30 seconds|
When SIGTERM or SIGHUP is received, the cache is put into "shutdown pending" mode until all active sockets are closed. This value is the lifetime to set for all open descriptors during shutdown mode. Any active clients after this many seconds will receive a 'timeout' message.