From memory, I think LineageOS 17 took roughly 8-12 hours for an initial build and 3.5 hours for subsequent (ccache) builds on an Intel i5-3570K and spinning hard drive. That's not including the initial git clone.
The idea might seem daunting, but assuming midrange hardware and a decent net connection, it's very much doable in under a day without resorting to cloud services.
I would expect it to scale pretty well, at least until you reach the limits of your disk and buffer RAM.
The build process supports the -j option just like make. You can use -j N+1 if you want to keep all your cores busy, or -j N-1 to keep your machine more responsive during the build, or nice and -j 1 if you're in no hurry and your machine has more important tasks. (Actually, I think reasonable defaults for these might already be part of the build scripts, but it has been a while since I looked.)
The idea might seem daunting, but assuming midrange hardware and a decent net connection, it's very much doable in under a day without resorting to cloud services.