It completely depends upon the subject matter and whether the writer has a good command of the written English language. I have a pretty high threshhold for reading long posts, and I am known as one of the long answer posters, because often there is plenty to say about a particular subject.
There have been a few instances where I have read the first sentence of a long post and do not continue, because the poster is giving a bad/wrong/misinformed/rant of an answer, but it wasn’t the length of the answer that was the problem, it was the answer itself that was the problem LOL.
There was one recent post on a Q, in which I can’t recall the subject, where the OP himself wrote a post in single spacing that went on longer than any post I’ve ever seen on Fluther and the beginning of it was rather nonsensical, so I just walked away (even though I was curious to know why someone would make a post like that) It was almost like it was written in binary code, but just happened to be using actual words, but it was still not understandable. It was the person who was asked to stop asking multiple, multiple, multiple questions in the details of their Q’s. Forgot who that was.
I’ve even been willing to read super-long posts where someone was describing a scenario, with a bunch of players, and a multitude of different situations were happenening and how all of these things were somehow inter-connected, but written very badly by the OP. I would go in and give the players names, Mary, Bob, John, Elaine, Grover, Delbert, Penelope and so on, then I would ask questions to try to find out who did what to whom, and when. Those Q’s are actually kind of fun.