WCF - how can I track response duration programmatically?

WCF - how can I track response duration programmatically?

client calls method "Foo()" of my web WCF service - and receives large
byte array as a response:
public byte[] Foo()
{
return new byte[10000000];
}
obviously, when the client reads all data, HTTP connection is closed - and
if I know when did this happen, I could track the total duration of the
"transmission". I know about Tracing etc - but I need to have this data
programmatically, so I can display it to the user. How can I track this?