Like radioactive atoms, words have a half-life. Or rather, they have a recharge time, which is the time (or length of text) from you use it the first time til you can use it again without vexing the reader.
Basically, when people read a book, they are not very conscious of the actual words and letters – they generally focus on the meaning that the words convey. But if the writer starts using the same words too much, the readers are torn from their state of absorbtion, as they suddenly become conscious of the actual words – something that also happens with grave spelling mistakes. To me, ignoring the rules of reusability is a sign of sloppy writing (disregarding the times when writers use the effect intentionally, as in poetry).
The most ordinary words have a very short half-life; they are highly reusable. Words like ‘you’, ‘me’, ‘man’, and ‘dog’, for instance, can be used constantly without fear of disrupting the reader’s mental flow. Only in the cases where these words are repeated – like the constellation ‘had had’ which is legitimate, but habitually avoided by editors – will they risk perplexing or annoying the reader.
Most other words are somewhat less reusable. ‘Subtle’ needs a few paragraphs, maybe a few pages, before it once again passes below the reusability radar. You can use ‘flummoxed’ or ‘insidious’ more than once, but there has to be a good bit of space between them. ‘Lugubrious’ needs at least a chapter, if not more. The rule seems to be that the more unusual the word, the less you can repeat it. (By the way, there is probably an interesting conclusion to be drawn here involving Zipf’s Law, but I’m not sure what it is.)
One of the best low-reusability examples I know is the word ‘nadir’. Nadir, normally used figuratively to describe the low point of something, i.e. ‘the nadir of my high school years’, is a beautiful expression. (Its better-known opposite is ‘zenith’, the high point of something – both words come from Arabic, from the field of astronomy). But use nadir two times in the same book, as it occurred in a story I just read, and it hits the reader with all the linguistic subtlety of a truckload of bricks. It seems, well, clumsy. Nadir is just one of those once-a-book words.
The same goes for some idioms and self-crafted expressions. On page 14 of the otherwise excellent book ‘Stiff’, telling the story of what happens to the human body after we die, the author Mary Roach uses the expression ‘a conversational curveball’ to good effect. But 160 pages later, when she describes something as ‘a philosophical curveball’, it stopped me dead in my tracks. Using the curveball metaphor twice, even that far apart, seems like an editorial oversight.
Jasper Fforde, an author who does interesting things with the English language, thought up a new device for use in literary settings, which he called an echolocator. It was a tool that scanned texts and warned the editor if the same word was used twice with 30 words of each other. I don’t know why he chose the number 30, but in the same vein, it could be interesting (and utterly pointless, but interesting things often are) to create a quantitative index of the reusability of the words in the English language.