DIsplay Block content after certain time delay
Create a block and set it to the region and page you'd like it to appear at
Use CSS to hide the contents of the block
Use JavaScript to override the hiding and display at a specific interval


The CSS would look something like this:

.block-DELTA {
  display: hidden;
}


And then just use JavaScript to display the block after a specified interval.  You could also add some animation to your CSS property to make it fade in.  (That's just one example tutorial on animating, google and you'll find hundreds more)

You can add the CSS and JavaScript to the block in a number of ways.  The quickest and dirtiest is in the block content itself.  I'd strongly recommend storing any code in either a module or theme.  See drupal_add_css() and drupal_add_js() for more information on implementing.