How long would a saltwater fish live in freshwater before dying?

not more than a few hours in the best cases, most of the times a few minutes

depending on the hardiness of the fish, i’d think no more than a couple of minutes for either fresh to salt OR salt to fresh.

basically what happens is the fish is used to the salt content (or lack thereof) and somehow uses it with it’s breathing the H2O. what happens when you put one into the other is they more or less suffocate because a freshw. fish cant handle that much salt, and a saltw. fish doesnt have enough of it.

my sister/bro-in-law have a saltwater tank with a dwarf lionfish in there. they feed him a couple of 10 cent feeder goldfish from walmart each week. if he doesnt seem interested in the food, they have to get the goldfish out and back into freshwater, or they die within 2-3 minutes. and goldfish, being part of the ‘carp’ family are pretty damn hardy fish!

so. that’s all i got for ya.

Depends on species; for example Tilapia can survive, live and thrive in both the mediums. Some macrals (Surmai, Bangda etc) come to sweet water estuaries for spawning, so do some other fishes, others may survive from a few minuets to some hours in sweet water.

Violet Gobies and Mollies can go both ways. That’s because they live in estuaries.