“When did Protestants start to regect the idea of Mary’s perpetual virginity?”
If you count Tertullian as the first Protestant, somewhere between 160 and 220AD. The official Marian doctrines were established by the Roman Church 300 years later, at around 541AD.
So a long time.
I would opine the new protestants started rejecting these (and other) docrines with the advent of the printing press.
541=431
Sorry!
>>I would opine the new protestants started rejecting these (and other) docrines with the advent of the printing press.<<
You beat me to it...