The way that a visitor reads on the web, is so much different from the way we would read content on paper. Our eyes fatigue faster when looking at a screen, and we're also much more task oriented. We want to find the information as fast as possible, so people are generally more likely to scan content, as opposed to read it word for word.
The main rule of thumb: Keep pages short
A guideline to work within would be to keep your average page length around 2-3 paragraphs or 150-250 words. If people see an overwhelming amount content, they might decide to abandon the page before even starting to "climb the mountain."
What if a page requires a lots of content?
Naturally, you will have pages on your website that require more content. In those cases, just make sure you don't write paragraphs and paragraphs. You will want to break up your content with things like sub-headlines (see photo of the text editor), images, videos, lists. These breaks will provide rests for the eyes and also help user move through the entire page.