Loop 30 seconds

I am making a simple web banner in Flash Professional CC (AS3).

The file has following layers:

• Actionscript (A button covering the entire button with a clicktag code embeded in "Action"
• Picture 1 (Last for the first 5 seconds)
• Picture 2 (Last for 5 seconds after picture 1)

Now I however need to make this banner loop. In the publish settings I can press loop at it goes on forever - but I need it to keep looping for 30 seconds.
What code do I need to use in order for that to work? And where should it be placed? 

(An alternative is a code to make it loop 3 times, since its 10 seconds. But that limits me to not change the actual time of the looping)-

Hope someone can answere this