What will mark the end of the period of Acharonim? What in general demarcates rabbinical eras? Are these halachic rules or conventions?
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.