The actual migration, a day or less. Now there was a 2-week sprint for testing and validating backup and restore before doing the actual migration. At the non-engineering level (management and such) of course there was a review and consideration time as well.

Good question.

I'd also like to hear more detail (budget, time, people and skills) on real cases.

When I had research students I set a few of them doing studies on migration, degoogling, data "repatriation" (ick!) and all the things I think we now call "sovereignty". But there was lots of theory and precious few solid, documented studies. There were also hundreds of propaganda pieces and Google/Microsoft/Amazon shill pieces sowing disinfo.

Surely that's changed now and the economic realities are clearer?