Here is the command to test the Keep-Alive timeout of a website:

VHOST="www.google.bg"; time openssl s_client -ign_eof -connect "$VHOST:443" <<< "$( echo -ne "GET / HTTP/1.1\r\nHost: $VHOST\r\nConnection: Keep-Alive\r\n\r\n" )"

And here are the today’s results for some popular websites:

slashdot.org: 0s
LWN.net: 5s
snag.gy: 5s
yahoo.com: 10s
readthedocs.org: 10s
www.superhosting.bg: 15s
httpd.apache.org: 30s
nginx.org: 1m
en.wikipedia.org: 1m
famzah.wordpress.com: 1m15s
aws.amazon.com: 2m50s
www.facebook.com: 3m
www.google.bg: 4m
cloudplatform.googleblog.com: 4m
www.cloudflare.com: 6m40s
www.mozilla.org: 6m40s
www.tagesschau.de: 8m
access.redhat.com: 8m20s
stackoverflow.com: 10m
www.timeanddate.com: 10m
www.dreamhost.com: 10m
www.reddit.com: 10m
twitter.com: 15m