Magically feel? For me on long pre-test pacing efforts, it takes 5+ minutes of being around ftp for HR to reach steady state around 158-162bpm. Start conservative and it can take 15 minutes. Here is an ok but conservatively paced effort from 2+ weeks ago: Where did you ride OUTSIDE today (2022) - #60 by WindWarrior
Once HR is around steady state, the fun starts with 1+ minute increases in power (~10W increments) and if HR drifts up its a tell. Its all about being able to tightly control power and knowing that HR can lag by 20-40 seconds, so I need to push a higher power for 1+ minutes before seeing a HR trend. The longer I go, the more I can feel breathing (& HR) increase when going over, so by 20 minutes into a 30+ minute effort then I try and pay attention to breathing as well.