"Θα έχω γράψει δέκα βιβλία μέχρι το καλοκαίρι."
Translation:I will have written ten books by the summer.
It's not incorrect but really, seriously ambiguous. Without context, it could have either "until" or "by". "Until" indicating the the 'duration' while "by" meaning "will you have made your statement before that time". Sorry, very clumsy interpretation. We have corrected it giving "Will you have talked by then? as the preferred sentence " but left the "until" just in case someone remembers it and gets confused. They'll be informed that by is the preferred word. Thanks a lot.