Allegedly, it took him a month - https://www.princeton.edu/~hos/Mahoney/expotape.htm. But the idea had probably been growing on him for several years, while working on very similar projects.

I think this actually supports my idea of letting ideas grow on you before executing them.

Further it's a tiny subset of MULTIC project that he worked on for some time. Some of MULTIC design ended up in UNIX. One can argue UNIX's design began as far back as that where he was doing something else but pivoted in new project with weaker hardware.

Wasn't that all he was doing? Also, could be mistaken, but pretty sure he was Ken Thompson.