From what I can remember and search to refresh my memory, Gen 2 for sure. If you mean by how the generations are referred to by number, roughly around Gen 3.
Back then, there were fans who used to refer to Gens 1, 2, & 3 as the Color, Metal, & Gem Generations respectively. Some also referred to Gen 3 as the Advanced Generation. I even saw Gens 4 & 5 as the Jewel & Monochrome Generations, but such terminology was falling or have already fallen out of favor by then. Of course, there was always referring to the generations by the abbreviation of the combined games (RBY, GSC, RS, etc.).
As for where, I cannot remember if there was a centralized source for the older terms. Same for the numbered terms, but I wouldn't be surprised if Bulbapedia made that widespread as wikis were a new and rising concept around that time.
