How can I see how long takes from when the HTTP GET message was sent until the HTTP OK reply was received

Set your time display format to "Seconds Since Beginning of Capture." (View > Time Display Format > Seconds Since Beginning of Capture.) In the Packet List pane, right-click on the packet with the GET request, and select "Set Time Reference." You will see "*REF*" in the Time column.

Now select the packet with the reply. The value in the Time column is the elapsed time between those two packets.

Or use the latest development version of Wireshark (a buildbot build or the recently-released 1.9.1): with that Wireshark will calculate these things for you (thanks to bug 8287).

Hey, Use Jmeter.. Its a good tool for any HTTP XML/JSON testes.. Automation is easy once you learn it