With mine the onset was very sudden. Here is my heart rate variability chart from the Apple Watch around the time my ectopics started.
The heart rate variability spiked on the afternoon of 5th September. Then that evening I did Mary Austin -1. It was lying in bed late that night I when I then noticed the palpitations, and they were very strong. The recorded HRV went from around 40ms to 360ms. It’s stayed elevated ever since though it’s not that extreme. It isn’t a true HRV number any more, it’s being thrown off by the ectopics. It’s clear some thing changed then.