UPDATE: Hiro Mashima comments on when the manga will end: https://www.theouterhaven.net/2017/05/fairy-tail-manga-end-two-volumes/
The official website for the upcoming Fairy Tail: Dragon Cry film announced that Hiro Mashima’s Fairy Tail manga is currently in its final arc, meaning that the manga will be coming to an end. When it will end is up to Mashima, though. We do know that final arcs can still go on for a very long time (see: Bleach), so Fairy Tail may stick around for a bit longer than one may think. The manga is currently in an arc where the guild is battling the Alvarez Empire, including the Spriggan 12 and Zeref himself.
Another note that the website made was that the Fairy Tail manga currently has over 60 million copies in print worldwide. The manga reached its 500th chapter back in September of 2016. The Fairy Tail: Dragon Cry movie is slated to open in Japan on May 6. The series currently has one other animated film and two anime seasons totaling 277 episodes. There have been numerous spinoff manga series involving the likes of Mavis, Gray, Laxus, Gajeel, Wendy and many other characters. These manga caused Hiro Mashima to get his own magazine (twice) dedicated to nothing but Fairy Tail.
The story of Fairy Tail is described by MyAnimeList.net as follows:
A superficial but kind-hearted celestial mage, Lucy Heartfilia, joins a famous wizarding guild in Magnolia Town called Fairy Tail, where the members use their powers to earn rewards in exchange for fulfilling quests. Here she befriends an impetuous but faithful dragon-slayer wizard, Natsu Dragneel, who is scouring the land for his missing father. The two form a bond with fellow mages Erza Scarlet and Gray Fullbuster, and together they tackle daunting challenges, taking on several powerful allies and foes along the way.
In addition to this, a new key visual for the Dragon Cry movie has also been revealed which you can see below
