Never?

Testing has always been a game of figuring out how much is enough to produce a stable product, without burning massive amounts of engineering time with testing. LLMs have not changed that. They will never be perfect, and that is OK. Same as with humans, you need to define your own comfort zone for quality controls and accept that there will be bugs. Fixing those bugs will be part of your process. If you get too many, increase testing. If you get none, you just have not yet discovered what was missed.