"You are very kind, but I have no idea of returning into Devonshire immediately. My visits to Mrs. Smith are never repeated within the twelvemonth." (Ch. 15)

I cannot say how Marianne might misinterpret Willoughby’s statement (above) from Ch. 15. Mrs. Dashwood was initially extremely upset at Willoughby’s strange behavior and obviously had her doubts which she explained away to her satisfaction but additionally she has gentled a twelvemonth down (not to weeks) but to a while in Ch. 15 and months in chapters 15 and 16.

“he feels himself obliged, from his dependent situation, to give into her schemes, and absent himself from Devonshire for a while.” (15)

“persuaded as he must be of your sister's love, should leave her, and leave her perhaps for months, without telling her of his affection…" (15)

"our dear Willoughby went away before we could get through it. We will put it by, that when he comes again.... But it may be months, perhaps, before that happens." (16)