Timer for each player

[Kapricioza]

Hi, i'm looking for best solution to create timer, who is starting always when player is logging and stoping when he's leaving. I tried with this:

function timeout() {
  setTimeout(function () {
    //something to do
    timeout();
  }, 10000);
}
timeout();

But i don't know how to stop this timer's function. It's of course via server-side but how can I create timer for each players? I wanna do something for player when he's long enough time on the server in the same session. 

[Xabi]

Can't you just store the timestamp when the user logs into the server and compare to the timestamp when he leaves the server?