Nice article! One part of my PhD research was to capture what constitutes software development expertise and which factors foster or hinder its formation (see this blog post). I asked my participants what employers could do to facilitate expertise development, but I didn't look into how much learning should happen at work, and how much in the spare time. It would definitely be an interesting direction for future work to investigate how much spare time developers sacrifice to improve their skills.
Thanks! I've put your article on my reading list, I'm really interested in your findings.
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