I think all they said about the base (free) Stadia is that it will be in the Spring, but no date.
Right now you can only have a Stadia account if you had the Founder's Edition, if you had or buy the Premiere Edition that's still on sale, or if someone who had the Founder's edition sends you one of their 2 buddy passes that they got.
So unless you get a code that way, you're out of luck for now. There's still a lot of stuff they haven't finished yet... a lot of incomplete features. So I doubt they are going to open it up to everyone until most of that stuff gets corrected.
I thought I saw somewhere that it was February, but I don't remember where. Then again, could just have been a guess, seeing as end of February is when 3 months of free/bundled Pro ends, give or take.
@Stoked4Good Yeah, I feel like they said "Spring" or March. But that goes along with what you just said. The first 3 months ends at the end of February, so March would make sense. It'll be interesting to see how it all plays out.