When applying to a high school, how long is an essay supposed to be?

I am applying to a high school and I am supposed to write an essay describing my single most important achievement to date. It doesn't say how long it is supposed to be.

For example: a five paragraph essay, short essay, etc.

Well, the directions read: Please write or type your response.
Update: My most important achievement, by the way, are my writing skills. Can you think of a thesis?
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