Hey Kev, dont judge you rear brakes by how much parking brake you have left, or by cable adjustment. The parking brake should be adjusted properly at the caliper, not the cable. As for how much you have left??? I cant see it from my house......

. But you need to determine the difference between pad and backing plate, there should be no less than the thickness of a piece of carboard left on pad. If it is thinner than this replace them.
If ya wanna snap a picture of them and send it to me, i may be able to make a determination, and if you are feeling froggy, i can talk you thru replacing them.......
fREak