When Should Numbers Be Written Out or Spelled Out in Full?

For general writing, most guides agree that you should:

• use words for the numbers one through nine, but for larger numbers the rules vary wildly from style guide to style guide. Check with your instructor to see which method you are using;
• spell out all numbers beginning a sentence; and
• never add an "s" to the end of the expression of quantity.  For example, write "ten thousand" and not "ten thousands."

A separate resource exists on this site that discusses How To Format Long Numerals (Numbers).

The following resources go into much more detail.  They instruct not only on when to write out numbers in full, but they also show how to spell out numbers in full.

Resource	Comments
TERMIUM Plus®: Numerical Expressions ★★★★★	Canadian.
Your Dictionary: Rules for Writing Numbers ★★★★☆
OWL @ Purdue University: Writing Numbers ★★★★☆
The Blue Book of Grammar and Punctuation: Writing Numbers ★★★★☆
CCC's Guide to Grammar and Writing: Writing Numbers, Writing Lists ★★★☆☆