I think under normal circumstances they last the lifetime of the bike. I have 16,000 miles on mine and they do not look even close to worn out. And I use the rear brake all the time in combination with the front.

i use my rear brake fairly heavily. went through a period of 2k miles where i used it almost exclusively because my darn front brakes squealed like a stuck pig.

at 20k miles, still plenty of life in them.

there is a wear limit indicator that will give you a ballpark idea of how much shoe you have left, not that it is really all that accurate or trustworthy.

Ive got 20,000 on mine, & ive used a grinder to remove a lot of the surface area,, still got plenty of meat on them.
Id be shocked if you ever need to replace them/

Unless you are an unbalanced brake user, I'll fall right in line with the other gurus here,... the rear brakes last a long time.  I started riding when I was 13.  I'm about five times that now, and I have never replaced any rear brakes (shoes OR pads).