Amber brake warning light

Anybody recall at what point you get the amber light (amount of pad thickness remaining)? At ~17,000 miles and it came on a few days ago. Hard to tell with out removing the wheels and I am currently without jack...

This is a good question. I just had my warning come on over the weekend. 2013 with 36k miles. Amber brake warning on the dash and "Brake Pads Worn" message on info screen.

How much do I actually have left?

I seem to recall it is 1-2mm, but curious if somebody here knows a specific value.

Does LR use replaceable wear sensor inserts in each caliper?

In theory, the sensor should go off when there is about 25% of brake pad left, so it should give you ample early warning - if everything is hunky dory and by the book... In practice, those things are far from being an exact science (the sensor is literally a looped wire that's supposed to wear out and break as it contacts the disc, opening the circuit and triggering the orange warning lamp in the instrument cluster), so keep an eye on your pads' actual wear at every scheduled service, tire rotation, and so on, instead of relying on the sensor alone.

The main issue is that there are only two such sensors in the car (mounted on one of the outer pads of front and rear brakes), so if you have uneven pads wear for whatever reason, odds are the sensors won't catch that until it's too late.

There's a sensor on the front left brake and the rear right my 2011 LR4. Assuming it's the same across the V8 range.

As Umbertobob pointed out, that leaves the other two wheels unmonitored, and it's not easy to eyeball pad wear without some crawling around. At least the the aftermarket pad sensors that I got from a local auto parts store for $20 are working just fine.