Every English teacher cautions against using run-on sentences. It's the writer's job to totally ignore them. A little extra prose is to be expected in novels, where description reigns and dialogue is meant to sing. But there's always a line that any author can cross. Are your sentences too long...and do you know how to tell?
Novels are supposed to be descriptive. You are supposed to literally paint a picture, only with keystrokes instead of brush strokes. It's not always easy to find the right words to use to describe events, places and people. It's even harder to put those words into the proper structure, and long sentences are a perfect example.
She looked out over a horizon painted in shades of red and gold, an endless sea of color in hues of danger, a warning that she was running out of time and the bandits were drawing closer and closer.
That sentence is pretty descriptive. It's also too darned long. Many authors struggle with finding a cutoff point, myself included, because they're trying to be descriptive. Basically, the sentence above describes a woman standing and looking at the sky. It only describes this one action, so it seems logical that the sentence should continue until the action is complete.
But it's not. Even if you're describing a single action, you have to break your sentences up into reasonable chunks. Look closely at your long sentences, and you'll find the cutoff points. You can find them in the example above, too:
She looked out over a horizon painted in shades of red and gold. It was an endless sea of color in hues of danger, a warning that she was running out of time. The bandits were drawing closer and closer.
Drop in a period, re-word a few things, and you can create cutoff points in all your too-long sentences. According to the strict rules of proper English, sentences are supposed to contain one subject and one predicate. The predicate is the action. By this rule, She looked is a proper sentence. In some cases, that can actually work as a whole sentence:
Missy pointed at the sky, screaming "look!"
But you couldn't write a whole book this way -- not a very pretty one, anyway. Many authors tend to write in compound sentences, which may contain multiple subjects and corresponding actions.
She looked where Missy was pointing, and felt a ripple of shock when she saw the clouds above their heads.
In the above, the subject is looking and feeling, and Missy is pointing -- all sorts of mess is going on. The sentence is a bit long, but not excessively so. It's pretty normal, as far as sentences in novels go. The first and last halves of the sentence could each be a single sentence:
She looked where Missy was pointing. She felt a ripple of shock when she saw the clouds above their heads.
But the very last part of the sentence, when she saw the clouds above their heads, cannot be a sentence on its own. If you wanted to break things up differently, you could use this fragment to begin the next sentence in the story:
When she saw the clouds above their heads, she knew nothing would ever be the same.
How Long is Too Long?
So, you know how to find cutoffs. How do you know when you need them? Just how long is too long when it comes to long sentences?
Seventeen words. No, that's a joke. There's no exact formula for sentence length, because everyone writes differently and some words feel a lot longer than others. A sentence is too long when it becomes clumsy. Every author writes with a natural rhythm of words. Some break this rhythm on purpose, jarring readers with a short sentence or an exclamation every now and then. Wow! Some slow it down by writing longer sentences, but once it starts feeling unnatural and becomes too much to digest it's just too long.
When is a sentence too long? When it's asking readers to absorb too much information at once:
She walked forward with murder on her mind, the leather strap wrapped around her right hand as if in preparation for the the dark deed ahead, her long skirts brushing the ground as she moved down the wall-worn path that was known to be used by the Evil Ones.
There's a ton of information in the above. I'm meeting a murdering female, I'm finding out about her weapon, I'm being given a setting and I'm even getting introduced to the bad guys. There's even some data on her wardrobe hidden in there somewhere. That's too much stuff happening in a single sentence. My mind can't absorb all of that at once. Now I have to go back and re-read, and now the flow of the story has been interrupted. If this keeps happening and my flow keeps getting interrupted, I might get frustrated and stop reading altogether.
Read your sentences. They're too long when you start to get bored with them, or get confused yourself and have to read them twice. If you're struggling with a sentence and you're the author, just think how the readers will feel.