For reference, my original AirPod’s batteries (buds, not case) noticeably declined around the 18 month mark.
I persisted with them for another 2 months before I spoke to Apple phone support, who then created a fault ticket for me to bring them into an Apple store, who then replaced both buds at no charge (due to Australian consumer law, which adds a 2nd year warranty on devices such as AirPods).
I’d expect similar degradation in the AirPods Pro batteries, so covering yourself for a second year would be smart.