Import a still image in a Bin.  It doesn't matter what the clip is.  To make this easy, you may want to configure your import settings to import still images as 30 seconds.

Edit the 30 second to V24.  Yep, that's right. 

You can add a V24 without adding all the tracks in-between by holding down the option key when you add a new video track.

So now you have a timeline that looks like this:

Next, right click on V24 and choose Lock Tracks. This will prevent V24 from changing.

The last step is to hide V24.  You can do this by navigating to the timeline menu and unchecking V24 from the Show Tracks menu.

That's it!  By heading a locked track, Media Composer will always display 30 seconds of filler.  This will make it easy to determine how much content you need to complete the spot.

We teach tricks like this in our Media Composer 101 Training Class!  Remember, it doesn't matter whether you are new to the software, or have years of editing experience.  There are always new and creative ways to use the software.