To specify a delay i.e when to begin set the begin attribute parameter in an appropriate media tag:
If you add begin="2s" in the cardiff image tag, you will see that the Cardiff icon will appear 2 seconds after the document began and will remain during 6 other seconds. Have a look at the source:
..... <body> <body> <img src="cardiff.gif" alt="The Cardiff icon" region="cardiff_icon" dur="6s" begin="2s" /> </body> .....