I do not believe there are any electronic sensors to alert you of brake pad replacement. There are physical squeal tabs on the pads that will "touch" the rotors and squeal when the pads are worn down to replacement level. You will hear the squeal when you apply the brakes.

 

Would you or anyone for that matter happen to know "approximately" what percentage is left on the pads when the worn pads finally reach the sensors??

 

When it happened on my wife's Optima, they said to keep speeds down, use the brakes as little as possible and I was told I was pushing it to try to get another 1000 km out of them ... and that was country travelling, not stop-start city travel.

 

When our 2012 Sportage started making a squealing noise, we had no idea what was causing it. It seemed to me that maybe a pin hole had formed in one of the many hoses that adorn the engine compartment.
However when we eventually realised that it was the brakes (it made a grinding sound when the brakes were applied,) we took it to a brake man who then had to replace both the rotors and the pads. I do not know what percentage is left on the pads when the squeal starts but I can almost guarantee that you will need to replace the disc rotors as well.
Good luck.0

 

For most any vehicle you can buy pads that have the squeal tab, and you can buy pads without the tab. It wouldn't surprise me if some factory brake pads have the tab and others don't. If you don't have the tab then you will get very little squealing and worn brakes will quickly start making grinding noises and quickly wear the rotors to the point they need to be replaced. If you have a squealer tab on your pads then it should start making noise early enough that you have time to get them replaced. I wouldn't embark on a cross-country road trip with them squealing but a couple days of driving to work should be okay.

All that said, sometimes brake squeal isn't at all from worn brake pads. It often comes from the grease between the pad and the pistons drying out allowing high frequency vibrations to resonate through the brake system and sounding like worn brakes. If you go to the shop and they want to replace the pads have them show you the old parts and the new parts so you can compare. If dried brake grease is the problem it should be about 30 minutes of work and $5 in materials to quiet it down.

I always do my own brake pads, as long as you don't need new rotors then front pads are about as difficult as changing oil and on most cars I have owned costs $30 or less. Rear pads usually last through 2-3 sets of front pads and are slightly more work than front pads.

 

To add to ^^
It's easy to check pads condition when tire rotations are done... every 10K miles or every second oil change, whichever one chooses...

 

The squealer tabs are only on the inner pads, not outer. There is about 1-1.5mm thickness left when the squealer starts touching the rotor. If you do not clean your pads, bracket, and slide pins every 15K miles there is a very good chance an outer pad will stick or a slide pin will seize and the outer pad could wear faster.