Creating a Sample CD using iTunes

This article straight from Cupertino might help.
You would select 20-30 secs of silence in-between songs.

 

Hmm, I read it as the OP wanted a particular song to play for 20-30 seconds, then immediately move on to another song...

I don't know if there's a native way to do it in iTunes, but you could use Audacity to create 30-second snippets of the part of the songs you want in the mix, import your snippets into iTunes, then burn following the directions in the article above, being sure to set the amount of silence between songs to 0.