https://www.invene.com/blog/limiting-developer-turnover has some data, that aligns with my own experience putting the average at 2 years.

I have been doing this a long time: my longest running piece of code was 20 years. My current is 10. Most of my code is long dead and replaced because businesses evolve, close, move on. A lot of my code was NEVER ment to be permanent. It solved a problem in a moment, it accomplished a task, fit for purpose and disposable (and riddled with cursing, manual loops and goofy exceptions just to get the job done).

Meanwhile I have seen a LOT of god awful code written by humans. Business running on things that are SO BAD that I still have shell shock that they ever worked.

AI is just a tool. It's going from hammers to nail guns. The people involved are still the ones who are ultimately accountable.