You can't. FOP2 launches the action, but does not keep track of times itself. There is an audit log (command line option -a) that you can enable, but it won't give you a duration, just whenever an extension started an action. You might also check the CDR records and see if you can find a way to identify records involved in a spying session, probably using the app fields on the CDR table.
Comments
Indeed the cdr field lastapp table includes a call option ChanSpy every time you listen through FOP2.
Thank you