Quoted from BeaverBrewing:No scientific proof...but for game made prior to 1990 that isn't a "collector" pin I have heard that you take off 20% off the market for ten years...then 20% off that for the next ten and so on.
I would have expected any formula to start with higher percentage losses and reduce it over time. Because from what I've heard, many games got parted out or dumped by operators pretty quickly after they stopped making money on route.