let's say you turned 40 in 2021 and starting working in 2004. The average college grad salary that year was $42,108. So we'll take that x2 (assume you get married) then x3 as they suggest. To get that amount you'd need to save/invest $715.25 each month, assuming 6% growth. And that's also assuming you don't get any raises or lose money to inflation.
I am not arguing with what experts suggest, only saying that's it's much harder to do in reality than we think it is on paper.
We have 529s for both my kids - ages 6 and 3. If we stopped contributing now, assuming 6% growth, we'd have $90k for the youngest and $131k for the eldest. I better boost the youngest's contributions.