Years of school (reading, calculus etc) to get to the point of learning basics of set theory. One day to learn basic SQL based on understanding the set theory. Maybe few weeks of using SQL at work for ad hoc queries to be proficient enough (the query itself wasn't really complex).

For the domain itself I was consulting experts to see what matters.

I'm not sure that time it would take to know what to prompt and verify the results is much different.

Fun fact - management decided that SQL solution wasn't enerprisely enough so they hired external consultants to build a system doing essentialy that but in Java + formed an 8 people internal team to guide them. I heard they finished 2 years later with a lot of manual matching.