Being a thick talentless writer...

I am stuck once again on the way something should be written.

In trying to write the next chapter of Get A Life, I am in a bit of a quandry as to how I should write this:

We settled on tea and scones and after Mandy served us, she left Sally and I to catch up on all that had happened in the last twelve hours.

My question is, should I write 12 hours or twelve hours?

I have had this problem before regarding numbers and I am sure that some of you hot shot writers and or editors out there will put me right.

Hugs
Sue