I've read many different answers.
For salt prints I use 1% sodium sulfite for 3 minutes followed by at least an hour of washing.
I agitate the sodium sulfite gently and the wash is occasional agitation ( every 5 or 10 minutes ) in changes of water.
That's not based on any testing, just based on what I've read. Some of the early salt prints that have survived the best came from printers who tended to use long washes ( even days! ). A few kinds of paper can't take such a long wash: Strathmore 500 2-ply Bristol will separate, and some waterleaf papers will fall apart.