Ansel had recommended 5 changes of water while soaking for 5 minutes per tray change.
This is HT-2:
You can use Kodak's HT2 for a test of residual hypo - HT2 has 7.5 gms of silver nitrate and 125ml of 28% acetic acid and water to a litre. Place a drop on a blank piece of paper that has been washed with the other prints, flush with a salt water solution after 2 minutes. Anything more than a light yellow stain indicates residual hypo (i.e., insufficient washing).
I think this is attributed to our own Ed Buffaloe.