in Editing
Numbers in fiction: words or figures
Fiction usually spells numbers out, but "usually" hides a set of exceptions. Here is how to set a rule and apply it.
Numbers look like a small matter until a copy-editor starts counting how many ways a manuscript has written them. "Twenty minutes", "20 minutes", "twenty mins". "The 1990s", "the nineties", "the '90s". "Five o'clock", "5 o'clock", "5pm". Each is defensible. Using all of them in one book is not.
The general tendency in fiction
Fiction tends to spell out numbers far more than non-fiction does. Words sit more comfortably in narrative prose, and figures can look technical or report-like on a page of fiction. Many publishers spell out numbers up to one hundred and round numbers beyond that, such as "two thousand" or "a million", and use figures for more precise large numbers.
That is a tendency rather than a law. Some books use figures more freely, especially in genres where the characters deal with numbers professionally. The important thing is to set a rule and keep it.
Common exceptions
Even books that spell out most numbers usually make exceptions.
Years are almost always figures: "1984", not "nineteen eighty-four", except in dialogue where the way someone says a year matters.
Some identifiers are figures because that is what they are: street numbers, room numbers, flight numbers, page numbers, model numbers. "Flat 4" and "the 38 bus" look wrong spelled out.
Times of day depend on context. "Five o'clock" and "half past five" are natural in narrative. "5.30pm" may suit a character reading a timetable or a text message. The choice may differ between narration and quoted written material.
Measurements and money are handled differently from house to house. Some spell out "fifty pounds"; others prefer "£50". Both are common in fiction.
A widely followed principle is that numbers referring to the same category in the same passage should match. Under a rule of spelling out numbers up to one hundred, "guests aged from eight to 102" mixes words and figures for the same kind of thing. Many style guides suggest treating such numbers alike, usually by using figures for both.
Numbers at the start of a sentence are usually spelled out, or the sentence is rephrased.
Dialogue
Dialogue gives the writer more latitude, since speech is heard rather than read. Spelling out numbers in dialogue is the more common convention, because it reflects how the words are spoken. "Nineteen eighty-four" in dialogue tells the reader how the character says it; "1984" does not.
Decades have their own set of choices: "the 1990s", "the nineties" or "the '90s". Many British publishers prefer "the 1990s" in narration and allow "the nineties" in dialogue or informal voice. Note that there is no apostrophe before the "s" in "1990s".
Record the rules
Whatever you choose, put it on the style sheet with examples. The rules for numbers are more intricate than they look, and having them written down means you only have to decide once.