I wasn't able to leave my babies for that long until they were over a year. Even at 15 months, which is the youngest I have left either of them for that long it was tough!
DS was probably about two months old when I first left him...but that was for about 12 hours (overnight). When I first left him for what I'd consider an extended period of time (four days), he was almost a year old. It really depends on your disposition, though I would say my biggest concern in leaving a baby that small for that long would be a disruption in BFing (if that's what you intend to do).
Re: STM please..