If you've got an hour or so do dive into the weeds, here's the story of I-5 in California.
tl;dr: it was what HSR would call a "blended system" - existing highways and freeways connected with new sections, then upgraded to interstate standards. The route was mostly designated in the mid-late 1950s (with continued fighting over some details into the '70s), with initial construction to original interstate standards in the late 1950s. Major construction of new sections was mostly done in the 1960s-70s. The last section of the initial construction process was reconstruction of an expressway section of the old Route 99 in the Sacramento River Canyon completed in 1992. By then, of course, other parts of the route had been reconstructed and expanded at least once, and more is certainly needed.
So I-5, in essence, took about 40 years to build.
40 years is roughly the minimum it's likely to take to build a working HSR line anywhere between the Bay Area and LA, given the rate of progress so far (we're about 30 years out from the initial proposals, and Work In Progress so far is just a partial line in the Central Valley and electrification of Caltrain). It's a little more difficult for HSR because they started with so little that's useful, so maybe 50-75 years would be better as a projected construction period.
Similar things happened on several other roads. I worked on some projects in the Fresno area in the 1980s-90s that were originally planned (and some of the r/w acquired or maintained clear by local government) in the 1950s, but didn't get funded until Fresno County passed a sales tax measure in the 1990s. At that point, things got serious with environmental and design, and most of the things I had some involvement with are now built or under construction. The major pieces were completed by 2005-10. So again: 50+ years from when the fundamental decisions were made (and ~20 years from when the funding program was approved by the voters). And frankly, it's taken almost as long for HSR to poke a hole under 180 as it took to build 180 from 99 to 41 in the first place (once the r/w was cleared including a couple of historic building relocations).