[ghaff]

>Isn't this true for any skill

I'm not sure why it would hold that, if you do something for 40 hours a week, you'll necessarily improve if you do it for an even greater amount of time. And, besides, effectiveness as a developer is not just about coding anyway. A lot more goes into creating products and services than lines of code.

[jpd-]

The underlying assumption is that when you code outside of work, you're working on things that are different from what you do at work.