Well like any other animal, exposure to increased or lowered salinity levels would highly affect their lifespan.. whether it's a few hours, days, weeks, or even by years. Mollies and some other fish I forget are always exposed to rapid salinity swings enabling them to survive normally in anything.
In a hermit crab's situation.. they're very hardy but I think just cause their hardy and can live longer than a norm fish people begin saying they can live in brackish for a decent period of time.. not nessecary meaning they can live in brackish for their whole lives. my 2 cents anyway.