I've plugged in SNES cartridges that are 30 years old and the save data still persists. I'm guessing they got the same man who designed the everlasting Casio digital watch battery to design those.

My 33 year old final fantasy cart still holds saves; other newer ones have died. There's a huge variance there.

I was surprised last year to find my saves on Zelda 2 and others were still intact. Zelda 1 had been dead for years though.

I've heard about this for years, but I've never had it happen, and I don't know anyone else who's experienced it. I don't doubt it happens, but what could cause such a discrepancy in the time it takes to degrade?

Different battery manufacturers? Batteries stored in different conditions?

Anecdata

I recently got my old Game Boy games out so my son could play them. They've been in a box in storage for 20+ years. My saved games from the 90s are still there on all the ones I've checked.

Back them up on a old gameshark or something. I lost all my gamesaves and wish I still could revisit them.