Some rules about Numbers in writing

The rule is:

If a number is 10 or greater, use numbers, and for zero to nine, write it out. 

Thus write:

We have just 15 minutes until then.


You've got five seconds left.

Exceptions:

Never start a sentence with a number. Thus, write:

Fifteen minutes have passed.

Not

15 minutes have passed.

Also, this is a matter of style and preference, but I prefer to write out all numbers and abbreviations (except Mrs. for which there is no commonly accepted spelling) in dialogue. Thus, I would write:

"We turned onto one hundred and forty-second street twenty-five minutes ago!"

(Notice, hyphens were used. For the numbers twenty-one to ninety-nine, use hyphens to join the tens' place to the ones'.)


"Good morning, class. My name is Mister Collins, and I'm going to be your Algebra teacher for the foreseeable future."

Comprehensive rules, according to Sarah of EssayForum.com (her source being the Publication Manual of the American Psychological Association):

Use numbers (numerals) in these instances:

1. For all numbers 10 and above: "There were 17 students in class."
2. All numbers below 10 that are grouped in comparison to numbers 10 and above: "Ony 5 of 17 students passed the course."
3. When using numbers immediately before a unit of measure: "a 5-minute wait"
4. Numbers that represent statistical or mathematical functions or formulas: "a ratio of 12:1"
5. Numbers that represent time, dates, ages, sizes, scores, money, and points on a scale: "It happened 5 years ago"; "a roomful of 6-year-olds"; "$40."
6. Numbers that represent a place in a series: "week 7 of an 8-week diet"
7. In a list of four or more numbers: "We had 1, 2, 5, and 8 pieces, respectively"

Sources:

http://englishplus.com/grammar/00000125.htm

http://www.dailywritingtips.com/10-rules-for-writing-numbers-and-numerals/

http://www.essayforum.com/general-writing-questions-13/use-numerals-spell-out-words-694/