User blog comment:FairyRaven/Fairy Tail Chapter 375: The Supernatural, Review and Commentary/@comment-4718355-20140314143552/@comment-7418318-20140314171525

The GMG arc was a little less than two weeks long (unless you already count the three month that Lucy & Co spend in the Celestial Spirit Realm).

Wendy joined FT when she was twelve, but since we don't know when her birthday is, she could have turned 13 on any of the days after that.