whenever there's no more lining left on the shoe. the drums are replaced whenever they went out of spec (too big). Just get some off the shelf brake shoes. Most of the time it's the shoes that got finished first, not the drum. If both gone at the same time, check the shoes adjustment, might be too tight. But usually you'll be replacing the shoes by the time you got to your 2nd or 4th front pads. Most of the braking force is up front, the rears only assist.
I have 76k miles on my car, and I'm still on the original shoes.
Mine isn't the typical setup though... I've adjusted my rear brakes such that they engage only slightly, even at full-brake application. It takes 4 clicks of the Parking-Brake lever before the rear brakes will engage at all. That way, I don't run the risk of locking up the rears in the middle of a 100mph turn at the track. Under normal driving conditions, my shoes aren't even touching the drum.
So I'm basically riding around on front-brakes only, but I still have to brake-check people because I know my car will out-brake them in an emergency (No ABS either), and I don't like the idea of getting rear-ended.