Free spin expiry?

Noticed I had some free spins left over from last weekend but by the time I logged in this morning, they were gone. Guessing there’s some kind of expiry window on these bonuses, but it’s weird since some sites seem to let them sit longer than others. Definitely makes me wonder if I’m missing some fine print or if it’s just random based on who’s running the games.

Anyone actually know for sure how long you usually get before free spins vanish? Or is it just all over the place depending on where you play?