Siteimprove relates these word counts within a sentence to how difficult they are to read:
Fairly difficult (Medium long sentence length): 21-25 words
Difficult (Long sentence length) : 25-30 words
Very Difficult (Very long sentence length) : 30-40 words
Extremely difficult (Extremely long sentence length) : 40+ words
Studies from the American Press Institute have shown that when average sentence length is 14 words, readers understand more than 90% of what they are reading. At 43 words, comprehension drops to less than 10%. Long, complicated sentences force users to slow down and work harder to understand what they are reading. This is not something people want to do, even if they are familiar with the subject or language you are using. Similar research from the American Press Institute shows readers find sentences of 8 words or less very easy to read; 11 words, easy; 14 words fairly easy; 17 words standard; 21 words fairly difficult; 25 words difficult and 29 words or more, very difficult.
Martin Cutts, the author of the book Oxford Guide To Plain English, offers the following guideline: “Over the whole document, make the average sentence length 15-20 words.” And what is the reason? He explains: "More people fear snakes than full stops, so they recoil when a long sentence comes hissing across the page."
Note: Some readability algorithms (LIX, SMOG, Flesch-Kincaid, Gunning Fog) ignore digits such as dates and phone numbers, while others include them (CLI, ARI). Therefore, the same sentence might not be considered "long" for all algorithms. Learn more about the different readability tests.
Additional resources for Readability: