My project is around 600k lines of C++ code, with around 1000 source files. Not even half as big as yours, but for what it's worth my incremental build time is

My project is around 600k lines of C++ code, with around 1000 source files. Not even half as big as yours, but for what it's worth my incremental build time is a bit under 3 seconds (and is almost entirely taken up by linking). A full rebuild for me (on a Ryzen 5950x, doing the build in parallel across all 32 logical cores) takes about 80 seconds. I feel that for the creative projects I work on, having an iteration time under five seconds is the most important thing for me. Once I've written the code, if that code isn't compiled, linked, launched, and visibly running on screen within five seconds of when I finished typing the code, then I'm going to be tempted to check email or otherwise context switch while I wait for it, and it might as well have taken half an hour. I spend a lot of time early in development trying to figure out optimal approaches to getting that iteration time down, optimising for incremental build times, link times, and debug build launch times. I find it pays huge dividends, and quickly, for me.