When I use numbers, when should I spell the word as opposed to just using the number? [duplicate]

I've just read this in a review:

Thankfully, the energy efficient Qualcomm chip helped the handset reach a best-ever score of 17 hours and 30 minutes in our continuous video rundown test. Compared to the outgoing model, which lasted ten hours 43 minutes in the same test

The first part uses 17 hours while the second part uses ten hours. Which is correct? What are the rules to determine when which should be used?

A common convention (used by The Guardian style guide — see "numbers") is to spell out the natural numbers nine and below, and use numerals for natural numbers 10 and above, and for any non-natural numbers (decimals, negative numbers, etc).

(Updated: should be nine and below, not ten and below.)

Generally the guideline is to choose a small number (in this case 10), and write it and smaller numbers in full, and use numerals for larger numbers.

And be consistent.