Edited*** with updated info and pictures
On the CB it's really easy to see how much rear pad you have left
in this pic the gold color is the backing plate and the gray color with red arrow pointing to it, is the brake pad. (note how pad is lightly against brake rotor..this is normal). When new the brake pad is approx. 3/16" thick. When the brake pad gets within 1/16" or a little less of the backing plate it's time to replace (1/16" would be approx 1.5mm)
Here is a new CB1100 rear brake pad for my 14 dlx
Pad measures slightly less than 3/16" thick when new
Just replaced all the pads on my ST 1300 and I took them down a little far lol. Don't take yours down this far. Much further and I could have damaged my rotors. Knew they were due but kept putting it off. When I did get around to it, it was almost too late. I got lucky.