As far as I can remember, it was fairly soon. It's not something that is noticeable except under certain circumstances, at lease in my experience. My first set of AirPods Pro had the problem and I was not even thinking of returning them until I started noticing discussions here about the issue. I contacted Apple and they sent me replacements (I returned the originals). Then the second set began to do the same thing after a short time. I had an appointment at the local Apple Store to have my phone repaired, and I took my AirPods with me. When I was talking to the technician there, I mentioned it to him and he went and got a new set for me and exchanged them without much discussion. He knew exactly what I was talking about and did the exchange without an question whatsoever.
The new pair seem fine, I've had them for just over 2 months. I have not noticed any issues with them.