60. Arduino UART Buffer

What will be printed on the Serial Monitor?

void setup() {
  Serial.begin(9600);
  uint32_t milli1 = millis();
  Serial.print("12345678\n");
  while (Serial.availableForWrite() != 63);
  uint32_t milli2 = millis();
  Serial.print(milli2 - milli1);
}


void loop() {
}