Does anyone know what the specs of a new brake pad are?

In May 05 my rear brake pads were measuring 1.5mm and my front were 6mm. They replaced the rear ones at that time.

In September 05 my front pads were now at 3mm, so they replaced them. They said my rear ones were at 9mm.

If my front ones went down 3mm in 4 months, is that what I can expect to occur with the rear. If that is the case, since my rear are now at 9mm, in 8 months, they will be down to 3mm and in need of replacement.

This seems ridiculous. How long are brake pads supposed to last? I do live in San Francisco, where there tons of hills and a lot of stop and go traffic, but is this what I can expect is going to occur?