Hajime Isayama has mentioned a few times about ending the Attack on Titan series, but there's no official statement about ending it in 2018.
One of the earliest was in 2013 when he discussed the series with another mangaka Hiroki Endo in November issue of a martial arts magazine Gong, he stated that the manga would end in 20 volumes. (Anime News Network)
Attack on Titan creator Hajime Isayama revealed that he aims to complete his manga in 20 book volumes.
Next year in 2014, in October issue of a literary magazine Da Vinci, Isayama was interviewed and stated that he initially planned to end the series in 16 volumes, 3 years since then. (Anime News Network)
In the interview, Isayama said that he initially estimated that he would end the manga after about 16 volumes, but extended his plans to properly depict the emotional narrative and the growing number of characters.
Isayama then added he personally wants to end the manga in about three more years.
Meanwhile, in the same year, the editor of Attack on Titan Shintaro Kawakubo was interviewed by University of Tokyo Newspaper, and he stated that the serialization was slated to end in 3-4 years. (Anime News Network)
In the interview, he discusses the popularity of the series, "The serialization is slated to end in three to four years, but Attack on Titan is a work that will be passed down from generation to generation for a decade — no, five decades."
Fast forward to 2017, in August issue of Bessatsu Shonen magazine, an interview with him stated the connection between The Marley arc and the ending. (Fuku-shuu's Tumblr, spoiler warning).
Now that the series is smoothly moving along, tell us your thoughts as you move towards the final chapter.
Isayama: The Marley arc has finally begun. I've been especially attentive towards my readers' opinions towards Shingeki no Kyojin. Many fans have been supportive, and this series has undergone much growth itself. So now I feel a great responsibility to conclude the story properly.
If Marley Arc is progressing as you had originally visualized, then the ending of the whole story…?
Although I'm progressing towards the ending that had been set before, my approach towards the ending itself has changed from the original plans. Because now I feel responsible towards the reader. I originally wanted to illustrate something similar to the film "The Mist."
Continuing in 2017, Isayama's last blog entry in December 2017 mentioned the celebration of the 100th chapter, and the ending came as soon as possible. (Suniuz' Tumblr, spoiler warning)
And then, thanks to everyone, SnK series reached 100 chapters this year. Thank you very much for reading until now! I wish you can follow this manga until the end of its story, which will come as soon as possible!
By the way, I was often asked about something like "didn't the editor's office say the story had been stretched?"
I know there is an impression like that, but to me, in fact, I don't think particularly in that way. If you ask me whether it is true, I'd rather say "if it gets stretched, isn't it very bad to the whole sense of completeness of the story?" I fear things like that will happen.
Same as other people, I also think it better to keep the length short… Although this is what I have in my mind, when I actually implement the ideas appropriately, the story gets inevitably long…"
(the quote was modified to incorporate the intended meaning from the author's typo on "calling" vs "reading")
Lastly, in March 2018, there's an autograph with Q&A session in Oyama. When he was asked about the series ending, he answered with "no specific ending date". (SnK News, spoiler warning)
On SnK's Manga's Ending Date: "Have you decided on when to finish this series? Have you ever considered timing its completion with the Tokyo Olympics (In 2020) somehow?"
Isayama: "I'm now working on the manga without any decision about the ending date. But [Kawakubo and my] shared idea is to finish it quickly. Five years ago, we decided to finish the series in three years (Isayama and Kawakubo laugh together)."
Apparently, there's a spoiler discussion on Reddit on October 2017 (predate the Q&A session in 2018) discussing whether the series would end in September 2018 or not. However, most of them were only speculations from existing interview and no official statement.
The predicted end to the series has been extended so many times already. Right now Isayama said that the expected end of the series is around September 2018, and I just don't see this happening.