On the rear drum there is a set of marks that indicates when you are due to rotate the actuator lever a star or two to keep the leverage and motion in the sweet spot in the leverage/rotation range.
How does it indicate "worn out"? It doesn't -- like the clutch rod it just indicates time to re-tune things a bit.
When the cam lobe located between the brake pad surfaces jumps over center -- then it is time to put your new shoes in.
The drill is, rotate the actuator lever on the star to re-optimize the lever/rotation/motion for the first time (10 years, roughly). At that point in time, start looking for a good deal on a set of brake shoes, preferably squeal free. Buy them and set them by for later.
Have them on hand so when you do a tire change out and you look and see you are about to cam over. Or if you actually have a cam-over event then you can put them in. Cam over events are easy to detect, the brake petal flops uselessly and does nothing. Not that this has ever happened yet to anybody around here, but eventually it may.
To date nobody much has ever worn out a set of rear brake shoes, although lots of us have a spare set waiting for it ....What will more likely happen to you is having a contamination event from from Slime or some other cause that makes your brake shoes "slick" and unusable. Then you can swap them out because you have a set on hand and you don't want to be bothered about trying to clean the old ones.
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Rear brake shoes on new Savages are a bit abrupt and harsh, this is due to the intentionally rough surface finish that was left on the drum surface. They squeal a lot too.
Lots of miles on the drums and the surface finish smooths up some, and the braking becomes more squeal free as the shoes slowly seat to match up with the drum radius.
Would it bum you too badly to be told it takes 3-4 years just to seat your rear brake shoes to the drum surface all the way?