Waymo still has to use "safety drivers". So far, nobody seems to have full self driving without "safety drivers". Which makes it useless. That's the big milestone to look for. When Waymo can get rid of the safety drivers, which they tried briefly last year, they're getting close to something useable.
Some products take a long time to bring to market. Xerography - first copy made in 1939, commercial success 1959. Although we're now 15 years after the DARPA Grand Challenge, so we're coming close to that 20 year wait.
Television - first broadcast, 1928, commercial success, around 1948. So 20 years agaon.
Roller bearings - Timken founded 1898, Timken bearings in 80% of US cars by the 1920s. 20 years again. (Although it took a really long time for them to convert railroads. First locomotive with roller bearings, 1923. In 1949, they were struggling to get railroads to put roller bearings on freight cars.[1] In 1991, roller bearings became mandatory for US inter-line interchange of freight cars.) Air brakes and automatic couplers only took 7 years, but that's because the U.S. Congress made railroads convert and standardize between 1893 and 1900.
What took a really long time, in post-1900 technology? (Before 1900, manufacturing infrastructure wasn't really ready for fast deployment.)