Hi Crysania,

The BFL book does outline a specific approach to cardio that only takes 20 minutes on cardio days. The actual type of cardio exercise is left up to you but the intensities to do it at are outlined in the book. It is a type of High Intensity Interval Training (HIIT). Here is a link to a thread in which Red1cutie outlines the program.
http://forum.lowcarber.org/showthread.php?t=162091

Although I personally don't think that a dance class would lend itself to HIIT very well, I don't think that means that you'd have to give up the dance class. Just like someone might be doing BFL but also enjoy playing tennis or skiing, etc.

You might be able to pick up a copy of the book at your library.

Arnie