: Spell out numbers before times, days, months? The general rule I follow is to spell out numbers < 10. Also when use numbers before measurement I don't spell out. For example: I bought nine

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This depends on your style guide. If you're following APA, all of your examples look good to me. My reasoning:

I bought nine apples: The number is under 10 and no special rules apply.
We need 5 mL acid for this reaction: It is written before a "unit of measurement", so it is written as a number.

Jack, you already did it three times!: The number is under 10 and no special rules apply.
It has been five months since she left me: This looks like an "approximate unit[] of time", so it's written as a word.

Here's what APA says:

Use numbers to express:

a. numbers 10 and above
b. numbers in the abstract of a paper or in a graphical display within a paper
c. numbers that immediately precede a unit of measurement
d. numbers that represent statistical or mathematical functions, fractional or decimal quantities, percentages, ratios, and percentiles and quartiles
e. numbers that represent time[*], dates, ages, scores and points on a scale, exact sums of money
f. numbers that denote a specific place in a numbered series, parts of books and tables, and each number in a list of four or more numbers

Use words to express:

a. any number that begins a sentence, title, or test heading
b. common fractions [one-fifth of the class; two-thirds majority]
c. universally accepted usage [the Twelve Apostles; Five Pillars of Islam]

(I got this list from here, which unfortunately forces you to download it.)

[*] The APA Blog clarifies here that for "approximate units of time" you should use words.

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I honestly think it is a matter of flow. Spelling out numbers is more intended for an audience you expect to read what you are writing whereas writing out the numeric forms is more for skimming. At least I have noticed that it is easier to skim through a passage that has numerals in it. They stand out among regular letters, and make them seem more important than the rest.

"Three days later." looks better than "3 days later." There is something about numerals that starts to seem rather like you are reading someone's chat message. If this is text on a note that a character is reading, the second option makes sense. Numerals for dates -such as years- makes more sense as they are much more laborious to read when written out, and can tend to take away from the flow of the story. "It was in 2018." reads better than "It was in two thousand eighteen." The numerals also give more indication of what is being spoken of. The second option could just as easily have been a reference to the amount that something had originated from.

In the same vein I agree with you that measurements make more sense as numerals than as written words because the number measurement is frequently followed by a letter based indication. With two letter indications for measurement one can get lost in the other as the reader moves through the text. "I filled the cup 5 ml." does not get lost as easily as "I filled the cup five ml."

I hope this take on it helps you.

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