How long is too long?

I'm curious as to when a posting becomes too long? I'm using my Assassin series as an example. Most of the posts have been around twenty thousand words, but the last was thirty thousand and there was a dramatic fall off of readers. Part two is has the most clues as to where this story is headed and I wonder if readers are turned off by the length? Part three is ready to post and it too is over thirty thousand words and I wonder if I should be posting as chapters rather than parts? Personally I like longer stories, but how long is too long? Arecee