Never "peaked" heartrate-wise on a bike, but back in the day did it countless times on tougher distance runs in the hills. Back then, morning resting pulse was generally sub-50bpm, and "max" HR I ever saw was in the neighborhood of 185-190bpm a few times, and that was only for brief stretches of climbs or passing other competitors. Can't recall heart rate failing to drop down to normal, sedate walking rate (~70-80bpm) within a minute or two, no matter how rough the peak rate. Assuming no dehydration, and assuming we didn't throw all caution to the wind and run far harder than we should have.
One "epic" run took everything out of me, and I spent the next several hours dehydrated and coping with the after-effects. Heartrate was above 100bpm for a long while, post-run, until I got hydrated and fed. But that was the one anomalous day I can think of, over ~15+ years of competitive running.
In my early 20s, there were a few runs we often did at a very hard pace, climbing over the ridges [at less than 1800ft elevation], resulting in pulse rates in the neighborhood of 175+ bpm, sustained of 155+ bpm. It was the rare run where the pulse would stay above 100bpm after two minutes. So, dropping ~80 points or more in a couple minutes was normal. Being dehydrated never helped, of course.
These days (many decades later), during relatively harder cardio, I often peak at ~145-150 bpm for modest periods, with sustained in the range of ~135-140bpm or so, dropping to sub-80bpm (after stopping) within a minute to a minute and a half. (A long way from going from 175bpm peak to ~80bpm, but not all that bad for an older fart.)