Is this the most natural way to put it:

I work in a big company. At 12 o"clock I have an hour break.

Or should we better say:

I work in a big company. At 12 o"clock I have an hour's break.

Or even

I work in a big company. At 12 o"clock I have a one-hour break.

I would go for the last version, being unsure in how widely the first two are used, under the "leave well enough alone" principle; but still I'm curious. Is "an hour break" okay?