Whenever you get this message "REMOTE HOST IDENTIFICATION HAS CHANGED" here public key fingerprint play important role. I wrote on this topic in this link
Now as per your question,you want to know when the known_hosts file was updated after trusting the key which means when the known_hosts file was updated with new public key fingerprint.
Here you have to check two files in target machine.
(1) /etc/ssh/ssh_host_rsa_key.pub (It keeps public key finger print information)
(2) Check /var/log/auth.log in Debian and Ubuntu, In Red Hat and CentOS /var/log/secure
(1) Get detail when the file /etc/ssh/ssh_host_rsa_key.pub get last modified. Below command will give you date and time stamp means when new public key fingerprint updated
(2) Now check in auth.log or secure file when you accessed the machine via ssh.
Hence,you have to find out when ssh access from client has happened after modify date time stamp of file /etc/ssh/ssh_host_rsa_key.pub. The nearest date time stap will be your answer.