In the days of hardening hypo fixers and before HCA Kodak literature for its Brownie and other rollfilms stated that if running water is not available, five changes of water five minutes each with agitation is sufficient. A 1930 Ilford manual stated 7 to 8 changes, 5 mimutes each with agitation, if water is short. Leica manual 1930s suggested 8 changes, 2 minutes each, vigorous agitation, if water is short. ( if you own Niagara Falls, all give the usual 20-30 minute running water spiel). Water is always short these days, and if not don't try to make it that way. Anyway, do these archaic methods hold true today if hardening fix is used? With nonhardening fixers would these be more surefire than the surprisingly fast Ilford method of today of only three changes?
And i totally forgot to respond to the OP - 10 water changes are all you need if you use a hypoclearing agent. In a nut shell.
I repeat! Use a retained Silver and retained Hypo test kit to test your wash conditions.
Maybe I'm a bit slow but who sells these kits? I tried a Google search and it didn't bring up anything I could match as being even remotely close to what you're suggesting. I've been meaning to actually invest in a Retained Silver Kit, but so far, drawn a blank. (Although TH does have it's drawbacks and nuances - so that could explain a lot!!)
The retained Silver test is Sodium Sulfide in water, and the retained Hypo test is Acetic Acid + Silver Nitrate. The first forms a black color on the paper or film and the second forms a yellow color.
I think that Freestyle, the Formulary and Fotoimpex all have these as standard stock items. Kodak and others have published the formulas for both along with color charts to give the user a hint as to his "quality".