All fish are different, their endurance can change slightly and some may hang in there longer. but it is fatal to use regularly. that damage wont go away and it will eventually cause other organs to fail or for the fish to become so stressed other ailments might surface, such as parasites and the like.
It just takes some time for it to show on some fish.
I would highly recommend IAL if you have yet to try them. when my bettas have issues they get some"spa time" and that helps build their slime coat and sooth any burns or fin rot they might have. The later usually at rescue time.
I would also strongly recommend buying a small kit, as in some situations, levels need immediate adjusting to avoid fatalities. Things we can't see are sneaky like that.
I truly hope things go for the better with your fish~