I trust the author can make it feel fresh and keep going some more but I rather a good ending now than watching this series become pokemon, if Ash wins the league then it's over, if Ash keeps the evolved pokemon he becomes too strong so he leaves them behind, he doesn't even grows old or remember the friends he leaves behind... This could be Akina (okay maybe she won't have 500 episodes but...) and I don't want that.
There is a quite obvious time progression in the series (counted from one Comiket to the next), so sooner or later, she will have to at least graduate from school. :D I think a more apt bad role model would be Sasameki Koto, which floundered around for many years before ever the author grew bored with it and put it out of its misery with the most anticlimactic ending ever.
And By The way, when did you started reading this? maybe the main reason why I'm so worried is because I'm here since the release of the first chapter 3 years ago! can you blame us for feeling disappointing after 3 years ? sigh ... Frankly I just want this to end around chapter 50, that would be great and for the new readers it would be excellent.
Three years is hardly anything to get worked about, on the grand scale. Some parts of the Nanoha fandom have been waiting for interesting plot developments since ViVid launched in 2009. :D But to answer your question, I think I got in on Shinozaki about a year ago, so I guess chapter 25-26?
But wouldn't any real change negate the very premise of the series and thus end it?
Only if the writer isn't good at their job. But if they're relying solely on formula, sure.
I am not sure what this author's record is (or whether they have much of one), so I cannot judge how good they are at their job. However, from a pure narratological perspective, once the core story premise is resolved, it must either be replaced with a new one, or the story needs to end. Personally, I find both options unsatisfying, as I quite enjoy the current premise at the moment and don't think it has been sufficiently mined for comedy yet. :-)