The rear wheel drum brake shoes may last several thousand miles, depending upon how hard or gently they are used.
Most of the time, I apply both brakes together. I like to use rear brake every time because it steadies bike. Occasionally it is used alone lightly if needed. Front brake does the major stopping. Just for reference, here are the pictures of rear drum brake shoes after 30,000 miles compared to new ones.
Even though they have lasted pretty long time and there was enough pad left, I changed them with new pads. Even when shoes do not need replacement, the drum is filled with dirt, grime, fiber powder and gets dirty. It helps to clean the brake drum/pivot points/ anchor plate from time to time for better functioning.