I don't think it's been confirmed how long her body has been out in the elements for, we assume it's two months because that's how long she was missing for but that may not be the case. Also, do we know yet whether she was clothed or not because that will make a difference to how much information of any kind they will be able to retrieve from her. JMO.
I don't have the link handy however one of the crew that found Jessica was interviewed and he said partially clothed. True we don't know how long she was at that particular spot. She was not hidden either and yet that is a busy roadway from what I have gleaned.