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Would it be all right if we said They are going to Canada from Late August to Mid December? That would make about three months and a half. Would that be all right? What do you think? I would really appreciate your comments on this.
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They are going to Canada from Late August to Mid December? That would make about three months and a half.

the first bit is fine - although we do not use capitals for the late and mid bits (some ppl might like a hyphen - but I dont really "get" hyphens!

The second half seems sort of accurate, but clumsy. I would say "That will be about three and a half months."
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I've checked a few dictionaries and books, and in all of them a hyphen is used after 'mid':
"mid-nineteenth century"; "mid-November", etc.
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Thank you for your comments: I really appreciate them. They have been most useful!
 miriam's reply was promoted to an answer.
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