However, already after the appearance of the first dual-core AMD Athlon64, 20 years ago, that time could be reduced to not much more than a half of day, while nowadays, with a decent desktop CPU from 5 years ago, most Gentoo packages can be compiled and installed in less than a minute.
There are only a few packages whose compilation and installation can take a noticeable time, of up to tens of minutes, depending on the chosen options and on the number of cores of the CPU, e.g. Firefox, LibreOffice, LLVM.
There is only a single package whose compilation may take ages unless you have an expensive CPU and enough memory per core: Google Chromium (including its derivatives that use the same code base).