Lots of really long sentences will tire your reader.

Readers have to keep all the words you have written in their mind until they get to a full stop and consolidate the idea. It’s easy to lose your message in a long sentence. 

Yet lots of short sentences can make your writing choppy.

The Australian Government style manual gives this advice: Include a mix of sentence lengths in your document – readers like variety and rhythm.

Keep sentences to an average of 15 words and no more than 25 words.

But there’s nothing wrong with an occasional 35 word sentence; nothing wrong with a 3 word sentence. Just don’t write them at that length all the time.