Try this, just paste it into Remotes (Alt+R)
Code:
on *:connect:{  
  .timer(autoaway) -io 0 10 scon -at1 awaycheck 
}
alias awaycheck { 
  If $idle > [color:green]1200[/color] && !$away { tnick [color:blue]$me $+ [away][/color] | away auto-away } 
}
on *:input:*:{ 
  if $away { away | nick $mnick } 
}

  ; Change 1200 to how many seconds you want to be idle before autoaway kicks in.
  ; Change $me $+ [away] to the away nick you like.