Shouldn't need to replace them for years, my brothers RT has about 100k on the clock and the original rear shoes are only about 20% worn.
Matt - they wont need replacing after 20k mate, more like 120k
*Edit* You should check em around every 36k, and while your at it give them a good clean with some brake cleaner, and check the lip on the drums. Also check the Wheel cylinders for leaks.