A quick Google search shows a lot of references to idea that it takes "20 minutes to get into flow state", but I haven't come across any original sources. If you know of any sources on this idea, I'd be interested in seeing it. I believe it from experience, but would love to see more literature on this topic.
Great post, Andrew.
Here are some relevant studies.
The Cost of Interrupted Work: More Speed and Stress
A Diary Study of Task Switching and Interruptions
That depends how far you want to go back :)
Wikipedia references Buddhist and Taoist texts that are literally NNNN BC. I would say though that what we call "flow" in a refined scientific sense was a product of "positive psychology" studies in the 1980s. Positive psychology is the opposite of diagnosing people with disorders, rather helping them find ways to avoid bad states and generally improve patient well-being.