What metric do you use to estimate remaining brake pad life on a typical economy sedan?

What metric do you use to estimate remaining front brake pad life on a
typical economy sedan (assuming normal driving under normal conditions)?

A neighbor's kid was heading off to college where her mother asked me to
take a look at her vehicle, where I found a few things to warn them of
such as this rack and pinion steering boot badly torn for some reason:
<https://i.postimg.cc/L4GHNsQG/boot01.jpg>

I told her that I had never replaced steering boots before so I wasn't even
sure how to diagnose whether she needed just a boot or the whole rack:
<https://i.postimg.cc/s20Fxntb/boot02.jpg>

One of the things I simply noted was they had "x" miles left on their front
brake pads (where there is only one mechanical wear sensor per rotor):
<https://i.postimg.cc/gJB3fdFn/pads01.jpg>

If you look closely, the pad is about the thickness of the backing plate:
<https://i.postimg.cc/pXjK1g0L/pads02.jpg>

The owner asked me how many miles those pads had left under normal driving:
<https://i.postimg.cc/nrD3HTJK/pads03.jpg>

I wasn't sure, so I guessed wildly at roughly about 5 thousand miles to go:
<https://i.postimg.cc/q7NyVq0y/pads04.jpg>

But what metric do you use to estimate a pad's remaining life given only
the thickness of the front pads, and the fact that you are told the vehicle
is driven by a typical owner under typical driving on typical roads?
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