Eyes get tired and beg for white space. You need to cover the topic in the paragraph, but seriously no one wants to read a paragraph that is 250 words long. I know when I am tired of a block of text, and I use that as a guide. If I am looking for a break, so are my readers. I know that is not a solid answer, but in writing there rarely is. There is no magic number of words or sentences. Eye fatigue is a real thing, which is why we also put pictures in our stories.