Disc rotor wear survey

I'm not sure you'll get meaningful results. I'd speculate at least 75% of the rotor replacements these days are unnecessary. I think the dealerships (and independent shops) have turned their backs on checking the rotors against specs (with micrometers and dial guages), just replace them. Ditto for rotor turning, way overdone.

Addendum: snapshots from attachment added:

 

i'm at 66k, and they look like brand new. both of my last two prius were still original into the mid 100,000's
admittedly, i'm a gentle driver, relying heavily on anticipated stops and regen. the most use the friction brakes get is a short yellow light, or emergency stop in fromt of me. but even then, i leave plenty of space

 

When I bought my 2010 trim V 15 months ago (61K on the clock), the seller let me take it to the dealer for one of their 'multi-point' inspections (~70USD). Among other 'things that needed to be done', they claimed that the rear rotors were out of spec; needed replacement. When I got back home, I measured the rear (and front) rotors with a digital vernier caliper. Front and rear calipers were dead on new spec (Front: 25mm-22mm minimum; Rear: 9mm-7.5 minimum). Dealer also said my CV boots were torn. They were in perfect condition...

As a prophylactic measure, I replaced all the pads and lubed the sliding pins.

 

I would use a micrometer instead of caliper: you want to take the thickness measurement in a little from the edge (say 10 mm), just in case there's a raised rust crust at the outer edge. The micrometer works better for that; the flat blades of caliper will be thrown off if there's deformity at the outer edge.

That said, it's probably all good, lol.

How 'bout a dial indicator and magnetic base? Christmas is coming...

 

There WAS a raised rust crust at the edge (and, Mendel, you think I'd actually measure the rust crust, really?).

I put two small bar magnets together, measured their combined thickness with the vernier caliper, then put the magnets on either side of the the rotor(s), then measured the combined thickness of magnets/rotor with the caliper, then subtracted the magnet measurement from the total to get the actual rotor thickness. Same (to spec) on all four.

 

yes, they all get the rust crust where the ads don't touch, and dealers like to point it out to gullible customers as proof of rotor replacement necessity

 

Good info above. One thing I'll add to the manual image there is that when Toyota says "Standard" thickness, they mean the original spec when new.
Even on normal cars without regen braking, replacement of rotors should be done zero or one time in the life of the car, typically. I doubt I will ever need to do it on my Prius. But of course dealers are happy to replace perfectly good stuff with new stuff - they'll do that all day long as long as people pay.

 

My front rotors (allowable wear 3 mm, from 25 down to 22) have 89% of the usable thickness left at 150,000 miles, so I won't expect them to quite last a million and a half.

Rears (allowable 1.5 mm, from 9 down to 7.5) have 82% of the usable thickness left, so just going by thickness, I'll expect those to be done around 850,000.

However: my rears have started with the rust crust (at both outer and inner edges). Those crusts may start where the pads don't touch, but they don't stay there, and have begun to carve the top and bottom edges of my rear pads. That's just a thing that started between last spring and now.

I could be tempted to haul them into a shop just to turn down the edge crusts. They would unavoidably have to cut somewhat into the remaining good thickness, because the crust ridges also have pits, which'll be perfect places for the rust to start again if they aren't removed.

So realistically, I'll probably never see 850,000.