I have been in between sort of a change from flowing water to standing water (as I have sometimes problems with water quality).
During that, I have monitored washing quality using HT-2 and to my very big surprise, there's no stain in the (sheet) film after only one or two water changes (15 liters per change).
After a fixing, I use a short rinse/wash using same method as with developing/fixing - rotation, tray or slosher, then I put the film sheets on the washing water and within 5-10 minute, I take the films off, change the water and put them back.
So far, after negatives have been on the second water bath for about 20 minutes, there's no visible stain left by HT-2.
To all my knowledge, I would have guessed that at least five or more water changes would be required for that. Perhaps it's the amount of water that makes difference?
So far, I do couple of water changes more just to be sure... But that finding is quite interesting to me.