Surely not even Marianne could misinterpret the statement that he made to Mrs. Dashwood? How could you take the sentence that he's never invited back within a year to mean he'll be back in less than a month?

And speaking of that - if she really believed that - why did she react as violently as she did? Why was there a need to canvas and recanvas her feelings and cause them to explode time and again if he was only going to be gone a few months? To me that really shows that it's not the feelings she truly has that matters to her - it's making herself look as romantic as possible!