I guess I should have gone to the source at the beginning. Apparently it doesn't matter so much how many rolls you put through it as how long it sits around. As a low-volume hobbyist, I will have to change what I have been doing - mixing up a gallon and using it one-shot until it's gone, which could be several weeks. Now it appears I should mix up only the amount I need for the largest development tank I plan to use and discard it in a few days.
From the Kodak Tech Info Date Sheet TI0688 -
Continuous and Rack-and-Tank Processing Maintenance:
Since the final rinse is not replenished and is further diluted by wash water which may contain bits of gelatin, slime or
algae may build up on the tank surfaces.
Under average conditions, the tank should be cleaned and refilled at least once a week. Conditions of high film
volume or poor water quality may require replacement of the solution daily. Allowing the tanks to stand empty overnight
will help eliminate the formation of algae. For persistent bio-growth problems, the tanks should be cleaned periodically
by treatment with a 5-percent solution of CLOROX or similar chlorine laundry bleach, followed by a thorough rinsing
with clean water.
Storing Solutions:
Concentrates of PHOTO-FLO Solution should last indefinitely if the container is tightly capped.
Because of the convenience and high concentration of the liquid concentrates, mixing of working solution just before
use is recommended. Also because of the high concentration of PHOTO-FLO Solution, "one-shot" use delivers the
highest quality final rinse while remaining economical.
Capacity
The surfactant capacity of a working tank of PHOTO-FLO Solution will depend in part on the amount of film put
through it, the quality of the water used in mixing, and the length of time a working solution is kept in service.
Generally, it is recommended that the solution be changed when it begins to bead, streak, or leave scum on the surface
of films or plates, or once per week, whichever is sooner.
Formation of slime on tank walls, and / or evidence of biological growth in the tank are also indications that the
solution should be changed more frequently.