Brakes are squeaking, probably have like 20% left on the rear ones (front ones were changed recently).

Should I wait until the car tells me to change? or just do it now because of the squeak?

 

I'd definitely do it when it squeaks. Nothing really can tell you that it needs to be replaced when the metal piece is touching the rotor. Sensors misread, malfunction etc. If it squeaks, it's time to replace then.

 

I wouldn't be so quick to replace. Brakes can squeak for many many reasons. I'd suggest popping off a wheel and taking a look - really easy to see how much pad is left (look at the pads on both sides of the rotors in case wear is uneven). The sensors seem to trigger with 3-4mm of pad still remaining. Of course possible they have been shorted across or not fitted properly so wouldn't rely on that. Always best to take a look

 

My replace brake warnings seem to be early, not late.

 

Mine were quite early as well.

 

I changed my brakes when the sensors told me. The pads still had about 25%. But the rotors had a sizeable lip. I am not sure if the metal on the rotors are soft or not. But because the pads still had lots of lining surface left, I was a little concerned. I replaced my rotors with slotted and drilled ones. They have been really good. The pads, at the same time, were replaced with ones made of organic material just like what came from the Jaguar factory. But the problem is that they squeak really bad when they are coming to a complete stop, especially if the rotors are cold or if they are wet from rainy roadways. They are really annoying and squeak only as you come to a complete stop. I have just ordered a complete set of semi-metallic pads. Hopefully this will cure the squeaking.

 

I had the same problem with squeak just as the car comes to a stop, more of a "graunch" really, on all three of my Jags - 2010 XFS, 2010 XFR and 2014 F-Type. Really loud, grating and annoying until the brakes got reasonably hot. Funny thing is that on all three cars if I shifted into Neutral just as I came to a stop there was no graunch, so I have been in the habit of doing this for a few years now, although it got a bit tiresome. I fitted Power Stop ceramic pads to the XFS which completely banished the graunch, I fitted Centric Posiquiet semi-metallics to the XFR and it reduced the graunch but didn't eliminate it (Power Stop don't make ceramic pads for the 380 mm XFR front brakes), and just this week I fitted Porterfield R4S "carbon kevlar" pads to the F-Type and now zero graunch and much smoother brakes. So I doubt if the new semi-metallic pads will cure your squeak, what you need are ceramic or carbon kevlar pads. So far I am really liking the new Porterfield pads but I don't know if they make them for XF. However I know for sure they make them for the 380 mm front brakes as the XF and F-Type 380 mm brakes are identical.

 

I have been contemplating the purchase of ceramic pads to see if they would work. Now that I know what works for you, I will do just what you recommended. There is not reason to do trial and error when someone else has already done it. Thanks for the tip.