Brake drums have a wear limit which is the maximum inside diameter the brake drum is allowed to be. This number is often cast on the outside of the drum it's allowed to be 422.1 millimeters or sixteen point six two inches. This is for your standard American trailer brake drum or drive axle drum. A brand new drum comes at 419 millimeters or sixteen point five inches.
My general rule is you can use two or three sets of brake shoes for every one drum. A good indication is feeling for a lip on the drum. A brake drum micrometer is by far the best way to measure a drum but if you don't have one of those this quick cheap and easy technique works well. Get a piece of wood or here I've used a piece of PVC pipe. Cut one piece at the maximum diameter 422 millimeters and cut another piece at the halfway worn point which would be 420.5 millimeters. Now by placing these inside the drum- straight not at an angle you can quickly tell approximately how worn your drum is. Be sure to check for out-of-round and bell-shaped drum. The maximum variation that I work off is one millimeter.