Problems Long Anime Can Run Into
Long anime tend to be the most popular with both casual and intense watchers. Think of shows like Bleach or One Piece. They're often common enough that even people who don't know much about anime will at least know the title of one of these. The best thing about them is that you get endless binge watching and it's a huge success when you finally catch up!
That being said there are a few ways that these shows can loose their audience. It can happen fairly easy once they hit a couple arcs in, but these are also ways that fans can determine if they should spend their time going any further with a series. Here are four very common problems they tend to fall into!
The Original Goal Has Been Completed

Think of long series that had one particular goal that needed to be completed. Perhaps it was to defeat a certain villain or obtain particular information. This can take a couple arcs, but when that goal is very narrow, long and drawn out story lines can get dull and boring to a reader. Not to mention that once that goal has been completed, there may not be a valid reason for the story to continue on.
That being said, this has to do with the actual writing and the nature of the obstacles put in place for our heroes. Consider shows like Bleach where things were initially interesting in trying to accomplish the goal of defeating Aizen. Once it was over, we should have been good. But continuing on after that fact can be a little dull because it's not really the story readers were initially reading.
Or They Didn't Have A Goal At All

This happens with a lot of monster-hunting like long anime. These are shows that start off with a character gaining some kind of power or knowledge about themselves that leads them to this goal of killing dangerous creatures. However, it's clear that the writers didn't have any kind of specific plot in mind.
There are plenty of anime out there that focus only on killing monsters or maybe even accomplishing these mini goals throughout their series. They can certainly be fun to watch, but usually they aren't the one's to get your heart racing. Think of the newer shows like Dragon Ball Super where the point is more just fight scenes; it has nothing compared to the original. When a long anime draws out their achievements just right, the earning of that goal feels so much better; remember that tension is important.
Over Powered Characters Are Mary Sues

A Mary Sue can really wreck an entire story. These are characters that seem to be overall perfect and all powerful. Once a character essentially becomes the most powerful being in that show's universe, there's no longer a story. Any obstacle put in their way suddenly feels like a waste of time.
Now certainly, with many longer anime, we might suspect that our heroes will win. After all, the telling of these stories are meant to brighten our day and give us hope. But what makes these stories interesting is how those characters overcome their obstacles. Shows like Naruto do this well when it comes to their villains. We really see our heroes struggle, but in a way that encourages us to continue watching and seeing those characters grow.
When Side Characters No Longer Become Important

Of course we all love our main characters! But sometimes it's nice to watch their peers grow alongside them. When the side characters grow as people within their world, you know the show is set to do well. This provides the audience with a sense of time and knowing that the world doesn't revolve around just the main character.
It also helps to compare what the main character, or even characters, are going through in their own personal arcs. If one of their peers went off for a while and came back, suddenly more powerful, we know that the training our character did may have been in vain or that there's more power to be found in this world. Plus, it shows just how much world building and effort the creators are putting into their actual stories.
We all love long anime. They can be the series that really keep us engaged or maybe even the thing we grow alongside. However, it doesn't mean that they can't have pitfalls in their stories that make us a little disappointed. When you find things like this commonly within your long anime, consider taking a break.