How long does it take to build a linux kernel on single core, 1GHz processor?

Around 130 minutes.

With modified kernel config to disable unused drivers and stuff, disable virtualization, tweak memory settings.
Everything on "Low Power Celeron M @ 1.0 GHz" (march=pentium-m), with 2GB of RAM and 2GB of SWAP on SATA2 HDD in IBM 4810-340 machine on latest (30.04.2021) Gentoo Stage3.