According to GCD, Superman #1 went on sale May 18, 1939. This would have been the 500,000 run I assume.
Superman #2 went on sale August 19, 1939 (no mention of a second print run that I could see). That's almost 3 months to the day.
So during that 3 months:
1. Superman #1 first print was released. Sales data was collected and it was determined that it was a runaway hit.
2. A second shorter run was issued with slight changes Sales data was collected and It too was a sell out or close to it.
3. An even shorter third print run was issued with slight changes. Sales data was collected...and by that time nearly 1 million copies had been printed and it was time for Superman #2 to be printed and distributed.
I would have loved to see how that communication chain ran considering it was 1939.