Depends on how dumb you are. Autopilots seem pretty dumb at this point.

16 to 18 years in most countries. Prediction and correlation of cause and effect, spartial awaraness and other cognitive functions take a while to train.

>how many years of experience is needed to drive on public roads without supervision

If, as I strongly suspect, full self-driving requires artificial general intelligence, Waymo's algorithms will not get there no matter how long they run simulations or even how many real road tests they do.