need to re-login

hi everybody .
     I have some question about re-login . In many web
that you have to log in before use, after you login and  do
nothing  in period time (about 1-2 hr or depend on setting) when you do anything agian .You have to re-login .
I would like to do that in my web but I don't know how to know
how long of client browser in status "do nothing" .
please suggest  me to do that.
thank you
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