'Set time limit for disconnected
sessions'
A "closed RDP Window" is in effect a disconnected session, so this is the setting to go with. Keep in mind though, that a user who suddenly loses WLAN ceverage and "falls off" is essentially the same, so set this high enough so that you allow users to log back into their session. A couple of minutes if you want to be strict, an hour if you want to be loose. Be sure to inform end-users of whichever policy you set, so that they don't get any unexpected results (I have some customers who assume that they can disconnect their session, shut down their laptops, go home, have dinner, and then reconnect into the same session. THis requires a longer disconnect time limit.)