what happens on Arduino power-on, and how long does it all take?
This is something that is far far easier to measure than to try and calculate.
Here's an Arduino Uno R2 with a simple sketch that just turns on D2.  Green is the power (7V to the barrel jack) and yellow is D2.

As you can see it takes 1.544 seconds (according to the resolution of my oscilloscope which, over those time scales, isn't that great) before D2 is driven high.
The sketch is simply:
void setup() {
  pinMode(2, OUTPUT);
  digitalWrite(2, HIGH);
}

void loop() {
}