12 lines/day/dev of "final" (tested, debugged, solves a customer problem) sounds about right.
The thing is - most code is not final. You end up writing exploratory code to test out a product, code that's rewritten because requirements change, code to test the actual code, code for projects that go nowhere and end up canceled, code for throwaway migration tools and other important tasks that aren't in the product but are necessary to build the product, code to make your job easier. All of that takes time, but it's not included in the metrics.
When I was at Google I used to joke that the half-life of my code was about a year, meaning that after a year, roughly half the code I'd written had been ripped out and replaced. Casual conversations with many other engineers indicated that their numbers were pretty similar. Over the course of 5.5 year career there, I wrote IIRC about 230,000 lines (there was a tool where you could instantly visualize your code delta across all projects you worked on). With a half life of a year, that's nearly 6 half-lives, meaning less than 1/64th of the code I wrote still exists in the codebase. It works out to about 3 lines of code/day, even though on an instantaneous week-to-week basis I was coding close to 1000 lines/week.
The thing is - most code is not final. You end up writing exploratory code to test out a product, code that's rewritten because requirements change, code to test the actual code, code for projects that go nowhere and end up canceled, code for throwaway migration tools and other important tasks that aren't in the product but are necessary to build the product, code to make your job easier. All of that takes time, but it's not included in the metrics.
When I was at Google I used to joke that the half-life of my code was about a year, meaning that after a year, roughly half the code I'd written had been ripped out and replaced. Casual conversations with many other engineers indicated that their numbers were pretty similar. Over the course of 5.5 year career there, I wrote IIRC about 230,000 lines (there was a tool where you could instantly visualize your code delta across all projects you worked on). With a half life of a year, that's nearly 6 half-lives, meaning less than 1/64th of the code I wrote still exists in the codebase. It works out to about 3 lines of code/day, even though on an instantaneous week-to-week basis I was coding close to 1000 lines/week.