For me it was when Intel introduced the Pentium Dual Core.
I think their clock speeds ranged from 2GHz to 3GHz.
Its been so long I can't recall the exact specs of the first gen dual core CPU's
I still have an Acer workstation from 12 years ago that was a dual core. Still works, don't remmber the number exactly but the spec was intel dual core 3700 maybe. 3GB of working RAM.
We started with IBMs, and I don't remember the specs, but we went:
Intel Pentium 90 > Pentium II 233 > Pentium III 450 > AMD Athlon 64 3200+ . After that It was a laptop for a long time, ultimately replaced by an Intel 2700k, which was then replaced by an AMD Ryzen 5 3600.
Actually multi-processing is fairly old. Check wikipedia Transputer - Wikipedia. Even suitable programming languages and dynamic core allocation where available already in the 1980. However the rise of MS and the "PC" in general made it more attractive to use the simple 16 bit Intel processors. 32 bit processors with linear address space existing outside the MS world when developers for Windows were still fighting the 64k barrier. To me it remains a mystery why the 8086, 80186... Pentium became so dominant.