What's a super-long sentence? One that is meant to convey a great deal of information to the reader, but instead of breaking it down into concise, easy-to-understand chunks, decides instead to put everything in one long string that might or might not be clearly broken up with proper punctuation, making the reader have to go back to the start and read it over again carefully to make sure they understood without missing anything, and thereby taking a lot of the enjoyment out of reading because it becomes more like a homework assignment. There - like that. :)
When you write a super-long sentence, you run the risk of two things. First, losing your reader halfway though. Sometimes when I encounter a long sentence, I've forgotten what we were talking about by the time I've reached the end. The other danger is that of reader fatigue. How tired did you feel after reading that sentence in the previous paragraph? Whether you realize it or not, you might be one of those readers who takes a millisecond mental break at the end of each sentence, and if you go too long without getting one, you might get worn out faster.
Not every sentence has to be super short. In fact, it's more enjoyable to read a document that has a wide variety of sentence lengths and structures. The brain likes to be kept active.
So, what's a good way to determine if your sentences are too long? I have two rules of thumb.
1. Can the reader easily follow your train of thought from beginning to end?
2. Can you read the sentence aloud without taking a breath, or are you gasping and panting for air before you reach the end? If you can't do it in one breath, chances are, it's too long.
Remember that reader enjoyment is our primary goal, and as we pay attention to our sentence length, we can create a story with better flow that our readers will appreciate.