What will mark the end of the period of Acharonim? What in general demarcates rabbinical eras? Are these halachic rules or conventions?
(Personally, I have heard from people that say that the Tekufas Haacharonim continues until Mashiach, and from others, that it ended a while ago, and that we are in the "Tekufas Rashei Yeshiva.)
In this article on Daat, they also say that we are still in the Tekufas Haacharonim, but note a number of variables that help us define the beginning/end of a "Tekufah":
כל תקופה נתייחדה, כידוע, בהיסטוריה משלה בעניינים רבים ושונים, כגון: תנאים חברתיים, גבולות גיאוגרפיים, השכלה, מאורעות היסטוריים וכיו"ב.
In this amazing article by Rav Moshe Lichtenstein, he suggests that a likely conclusion is that the Tekufas Haacharonim has already finished (p. 41). He bases this on his two major criteria, geographical/historical changes, and changes in learning style. However, he makes a very important point in his conclusion, namely, that we cannot predict the future, and we will only know this in the future:
סופו של דבר, מובן מאליו שלא הכותב ולא הקורא בעת הזאת יכולים לדעת את התשובה לשאלה זו, ורק בניהם אשר יקומו אחריהם יוכלו לקבוע ולהבחין מתי אכן נגמרה תקופת האחרונים. בינתיים אין ברצוני לעסוק בנבואות ואם לא בתחזיות אלא אך ורק להצביע על כמה מאפיינים של התקופה הנוכחית כנקודות למחשבה ולהרהור על מנת שנבין טוב יותר את מקומנו בהווה.
These therefore seem to be conventions, not Halachic demarcations.
(If anyone has access to Otzar Hachachma, I would love to read what Rav Shilat has to say on this in here. All I saw in the intro was that we are still in the Tekufas Haacharonim.)
I have once been told the answer to this is (although I have no proof) that the last one is the greatest of all. Samuel the last of the judges was greater than all of them. Rebbi the last 'tano' was greater than all of them. Rav Ashi the last amoro was greater than all of them. Rav Hai Gaon the last gaon was greater than all of them The Rosh the last rishon was greater than all of them. I would say the Chazon Ish the last achron was greater than all of them
So that tells when the end of the era is. If someone is greater than all the previous ones.
If not all the previous ones like Samuel at least the ones previous to them within 100 years or so. That would answer all the comments. Since someone so great comes along who doesnt really belong to that generation him being the only one that proves that this is the end of that era.