When the brain says stop
The brain seems to be designed to forget. Short-term memory can only take care of seven pluss/minus two elements of information* at any one time. And items can only be saved from 12 to 30 seconds.
We receive information much faster than the short-term memory is able to save it. The short-term memory is the main bottleneck in the reader´s ability to process information. As a writer you must take this into account.
Words of seven letters will dramatically increase the chance that the eye must read the word again. Many long words thereby increase the risk of your reader losing interest in what you have to say.
The more long words and sentences, the more the text is to read. Many people will not get the meaning. If you start using Greentext you can customize the text as to what readers are accustomed to reading.
* Source: Edmund B. Coleman & Gerald Miller.