Signs of failing brakes

[donut]

I believe the interval is 6 mos or 6000 miles. That said, I don't think my drums have been pulled in 2 years. (but it's not a DD) When pad wear is down to about 1/16"
Few indicators. Squeal/squeak from rear, e-brake no longer works when it used too, slight drop in master cylinder fluid level.

Rule of thumb I use and it seems to work for me. Every other pad change on the front, pop a drum. Assuming everything works correctly and the hardware is in decent shape.

Your rears shouldn't wear nearly as quick as the fronts, but a lot of folks just plain forget them until there is a problem.