2-4% of the time according to the former Cruise CEO Kyle Vogt in a Hacker News comment.

https://news.ycombinator.com/item?id=38145997

That reminds me of this, https://xkcd.com/1897/ perhaps it was based on something after all.

those grocery delivery robots you see in LA, turns out are being monitored/operated by people in a central location with all their camera feeds.