But the time for the emulsion to come to equilibrium with the wash water is independent of the amount of fixer left in the emulsion. Once the emulsion has the same fixer concentration as the wash water then it is time to change the water - soaking any longer won't improve matters one whit.
One never waits for things to go all the way to equilibrium, 90% of the way there is plenty good enough. I use 5 - 10 minutes for each water change for no particular reason other than it feels right, is convenient, and yields zero color on the residual hypo test.
4 or 5 changes of water - assuming a generous ratio of water to prints and some agitation and shuffling - is all that is needed for fiber base w/ KHCA.
A word on residual hypo test: the solution is silver nitrate, and unless all the silver nitrate is then washed out of the print it will stain as the print dries - this doesn't indicate there is fix left in the paper - turning brown in the light is what silver nitrate does.