I have a few 21.5t Helix RS, I run 21.5t blinky touring cars. One of the Helix 21.5t has done close to 400 packs in the last 15 or so months. Usually, I stick to ~48-49 degrees of timing with an FDR of anywhere between 3.4 and 3.7. The motor usually reaches 60-70 deg C (140 - 158 deg F). There was an occasion when the track temp was at 65 deg C (144 deg F) and I did not back off the timing nor the FDR. The max temp of the motor got to 95 deg C (203 deg F). The temps are from my ESC which stores the max temp.

I measure the rotor strength for each of my motors straight out of the box, and I have found that the rotors only lose a few percentage points of the magnetic strength (Guass) after a lot of runs under the conditions I have described. The motors still perform very well; in fact, there is no drop in performance, as seen from my Minipro dyno. One visible change is that the copper wire has turned to a purple-ish colour; however, there is no short, and the resistance numbers are comparable to when they were new.

This is just my experience, and your experience may vary.