Nothing wrong with a 2.5 hr film if it's warranted, I suspect a lot of the time that's down to personal taste. Personally I just try and enjoy films - sometimes they're overlong, sometimes 90 minutes I want more (that's better admittedly) sometimes I find 2.5 hrs perfectly fine - I don't see that there's any general rule you can put on it personally.
I think it's a very difficult thing for the film makers themselves to predict tbh. A couple of Costner films spring to mind. Dances with Wolves and Wyatt Earp - both are about 3 hrs, both essentially are atmosphere pieces - one is brilliant, one is so dull that I had to switch it off or chew my own leg off with the pure boredom of it (you can probably decide which is which for yourselves). I doubt he did that intentionally -sometimes you have to let the audience decide.
I can watch superheroes smashing the world to bits quite happily -----> moi <----- Philistine *shrug*
The Hobbit extensions have nothing whatsoever to do with Lost Tales, Unfinished Tales etc. Tolkien himself sold the film rights to the Hobbit and LoTR in his lifetime to pay a debt. Christopher Tolkien would not sell the film rights to the rest if his entire remaining family was on fire and despises Peter Jackson in particular I think. The filler in the Hobbit films is pure made up stuff by Jackson with a *little* bit of stuff from the LoTR appendices.
I can't image - amusing as it was in the final Hobbit film for a moment - that any Tolkien family member envisaged Dain Ironfoot as portrayed by Billy Connolly
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