Bantam silkies and polish here: the majority of my chicks pip externally on day 20. There are usually 1 or 2 early ones that pip on day 19. 90% of the hatching activity is on day 20, and the stragglers continue into the early hours on day 21.
Interestingly, this time around, I had three broodies, so I did a little broodie vs. bator experiment. I set half the eggs in my NR360, and then set the other half under the broodies 12 hours later (since, in my experience, the broodies always hatch faster). My two experienced broodies both started hatching around the same time as the first incubated chicks, but my first-time broody was a full 8 hours later. We are now early on day 21 in the incubator (late on day 20 for the broodies) and I have two eggs left in the incubator (both pipped) and two eggs left under the less-experienced broody.
So in summary,
Experienced broody hen: day 19-20
Inexperienced broody hen: day 20-21
Incubator: day 20-21
(pic of my experienced broody and her first hatchling yesterday)