This is basically impossible to determine based on how safe the car "feels".
e.g. if it has a 99.9% chance of doing your daily commute without crashing and without you intervening, you can monitor it closely from the driver's seat for 6 months and there'll be ~90% chance that everything looks fine and you never need to intervene. But then if you start napping in the backseat on your commute, there's a 70% chance you'll crash within 5 years.
There are enough people who FA'ed and FO'ed in the form of ending up dead, or killing others, that we're not stuck in abstract calculations or speculation.
e.g. if it has a 99.9% chance of doing your daily commute without crashing and without you intervening, you can monitor it closely from the driver's seat for 6 months and there'll be ~90% chance that everything looks fine and you never need to intervene. But then if you start napping in the backseat on your commute, there's a 70% chance you'll crash within 5 years.