You can do this by making the instruction into its own self-contained questionnaire (.que file). Then, in your experiment (.exp file), add the parameter (d-900) when lisiting this questionnaire. That will cause it to be onscreen for 900 seconds and the negative sign will cause it to be timed via a countdown timer. Hint.. You can use ctrl+right arrow to skip out of the item when running it as a test.

Hope that helps,
Blair