I recommend using the lightning speed Ichigo for the Soul King Palace distance:
Ichigo's casual speed after his training in the final arc
is faster than lighting speed, and his max speed needed for this calc is “unknown” so we have to use the speed of lightning for the calc as a low-ball.
But how did I reach the conclusion that Ichigo is lightning speed?
Ichigo's minimum shown speed is casually blitzing point blank lightning, after he arrived from the palace distance. It means that his speed cannot have changed.
Candice lightning is summoned from the clouds.
The light novels confirmed that even when the lightning girl (in the scan above), Candice's power got weaker (Yhwach took her Quincy Vollstandig which matches the Shinigami Bankai multiplier of 5-10x) still surpassed the power of any lightning in nature.
And it doesn't seem to be about the power, but just in general. Clouds and sounds of thunder are referenced multiple times. (There are also other feats/explanation for other character's abilities like Meninas and Giriko.)
In this other scenario, Tokinada a Captain who is weaker than Ichigo, reacted to the speed of lighting from a fodder Ninja from the second division who are not even captain level.
There’s no way Ichigo isn’t that lighting speed casually as shown in the scans, there is no outlier, and is fair to use.
I'll use 443842 m/s the average lightning speed accepted in VS Battle Wiki, based on the feat above.
"The speed of lightning can vary mostly in the range of 1.0-14×105 m/s. For the purpose of calculating speed feats we use the average lightning speed of 4.4×105 m/s (Mach 1294), which is suggested by this study. Similar studies have found average speeds within the same order of magnitude, albeit lower."
And before you say, “he rested” Ichigo training for a week in a treadmill designed to drain his spiritual energy and trained for months without rest.
Here is the accepted stamina thread for Ichigo's character.
Now that that is out of the way let’s make the calc.
CALC
Timeframe:
Normally it would take a week:
604800 seconds, but... Ichigo arrived earlier than expected by those who trained him, going at full speed.
He even crashed on arrival.
From the Clock Tower in Seireitei, and Ichibei's statement.
The Quincy take over Seireitei began at 5 pm the first day.
Ichigo went down after 3 hours at 8 pm
Ichigo arrived at 5:15 am the next day.
That's 9 hours and 15 minutes or 33,300 seconds.
distance = speed x time
distance =
14779900000 miles
Important note the distance isn't "realistic" is not a counter argument, because the distance is in between two different dimensions that can only be cross when the 72 barriers/doors that connected and open. Ichigo broke them all on his way down.