Years is literally just the plural of a single year. Ergo, years feels like the appropriate word here. What are you suggesting they phase it as instead?

I would always say “two” when talking about such a small quantity. “Years” is misleading, as evidenced by many other comments here.

“The battery lasts two years”

On this page [1] in the technical specifications under battery life, instead of actually saying 12-15 hours of recording or 2 years, it just says "Years of average use"

[1] https://repebble.com/index