This will respond with I am away [reason] [Gone: X mins]
Code:
on *:text:*:?:msgaway
on *:action:*:?:msgaway
alias msgaway {
  if (!$away) || ($eval($+(%,awaymsg.,$cid,$site),2)) return
  msg $nick [color:blue]I am away $+([,$awaymsg,] [Gone: $duration($awaytime,2),])[/color]
  inc -u[color:red]120[/color] $+(%,awaymsg.,$cid,$site)
}
Change the 120 to how many seconds you prefer between auto-messaging the user.