I have always spent time learning relational databases on my own, non-work time. As the years have passed, I have spent less of my own free time on technical learning and writing for many reasons, but still, I spend plenty of my own time learning about the database technology that Codd started.
While I never actually kept count, I have spent at least thousands of hours blogging, writing books, learning, and just playing around with SQL Server features to figure out how they work. I have also worked with some very competent people who only spent 40 hours a week