[Padarom]

There is an include which checks if the player is in the menu or not (if he's using alt + tab he will also get in the menu). I dont know it's name, but if you find it you can start a repeating 1-second timer and increase an integer for the player which is ment to be his afktime in seconds. Then you can show this via SendClientMessage, TextDraws, 3DLabels or whatever. When he get's back just kill the timer.