Well guys, I'll post it again. BTW, I have re-read Mason and Jacobsen along with the Kodak technical manual. And Dan, your common sense is not correct here. Common sense told mankind that the earth was flat too, and the sun revolved around the earth!
Here are the equations for washing:
Diffusion rate governed washing dX/dT = k(a - X) where a = thiosulfate in the emulsion, X = loss in concentration over T which is time. This is read as the change in concentration per unit time.
This is an exponetial when integrated as in:
k = 1/T * Ln( a / a - x) where Ln = Log base 2
So, if 80% washes out in 4 minutes, then 20% remains and 80% of that will take another 4 minutes. No matter what method is chosen, the residual silver complexes and hypo must have fallen to a level that gives the best image stability. Now, this equation only works at the instant of immersion when there is NO fixer or complexes in the wash water such as in freely running water with agitation.
As washing proceeds in still water or agitated non running fresh water, the equation becomes:
dX/dT = k[(a - x) - w] where w = the amount of salts built up in the wash water at any given time! The larger w becomes, the slower the wash becomes and in standing water (say any of the 5 or so changes you use) it becomes larger with time and is NEVER zero. In running water it can be made equal to zero.
In addition, with FB paper, this equation does not apply at all due to the cellular nature of FB paper and the washing is very sluggish and can take up to several hours. So, this works only with film and RC paper.
Mason goes on (as does Jacobsen) to describe the ideal wash being fresh water introduced into a final tank which overflows into the preceding tank and soforth for a series of tanks connected together. Jacobsen gives the diagram of this countercurrent wash which has been used for years by commercial photofinishers.
Kodak simply says "wash the film in running water for 30 minutes..... at a rate sufficient to achieve about 12 turnovers / hour...". For FB paper it is 1 hour.
Refernces: L. F. A. Mason of Ilford "Photographic Processing Chemistry"
C. I. Jacobsen of Pavelle "Developing"
Kodak B&W Darkroom Dataguide (2 suggestions - one for normal and one for archival use. I have quoted the archival above)