The six and a half minutes is only there as it is a doubling of the developer time of three minutes fifteen seconds. This is (I believe) mainly brought about by the plethora of Dip N Dunk machines which dip the film into a developer bath for 3 minutes, then starts raising the rack up, then moving the whole shebang forward, then dropping the film rack into the next bath with completion approximating to 3'15" give or take depending whether the film is at the top or bottom of the rack. For super critical stuff we would always have sheet film in the centre of the racks
The bleach and fix baths are double length baths, so the film rack goes up on the next cycle and is then dropped back into the bleach (or fix bath) for a second cycle; hence the 6½ minutes.
C41 is the shortest commercial developing cycle I know of, time in the first bath is critical for film speed and many other factors. A plus or minus of 15 seconds is around ½ a stop pull or push. The temperature at 37.7ºC (carried) is also critical for nailing things down. Although there is supposedly a real lot of give or take with C41 processing; for really on the money processing with good films, nothing beats perfect processing. The results of careful film exposure combined with careful film processing control, can lift your end result from pretty good to stupendous in a home processing situation.
For quite some time I was involved with Dip N Dunk C41 and E6 processes using Kodak chemistry and monitoring film strips etc. Sometimes when things went awry, the film racks would be lifted up, they didn't move forward and were then dropped back into whatever bath they came out of. Providing it wasn't the developer bath, the film would survive and within reason it could be there for some time. This happened at least twice on my shift in my time with these machines.
What I 'm suggesting is that if you can change the cycle length of your bleach bath to offset the possible weaker bleach solution this may also help you somewhat with your completion issue, if in fact it is an issue. Thinking out loud here.
For many years I was re-halogenating my C41 bleach solution using a simple fish tank aerator, then adjusting the pH back. I was rotary processing in a Jobo and using 500ml of solution at a time for four rolls of 135 film or four sheets of 4x5" film. There is always some solution loss in the tanks, plus evaporation losses due to heat, so around every 10 developing runs I would decant down to around 250ml add 500ml of fresh bleach then top up with the old stuff to 1000ml, discarding the used solution. Roughly every 20 developing runs, I would mix up a fresh litre of bleach and start all over again.
Short of running a Kodak C41 control strip through your diluted bleach and then doing a readout of the control strip, I think you'll be winging it. It would probably be alright though. If you have anyone doing commercial C41 nearby, they should have some kind of control monitoring system, worth a shot anyway as you have nothing to lose by asking.
That's excellent information. Thanks.
Also, koraks, thanks for your comments. They are encouraging.
I found some information on Kodak's document titled "Using KODAK FLEXICOLOR Chemicals" that seems relevant. On p. 3-6 it gives steps and conditions for rotary tube processing, and it lists 6:30 for the bleach time, but it give a temperature range of 24 C to 38 C (74 F to 100 F). Most chemical reactions are a lot slower at 24 C than at 38 C, and since the bleach will work at 24 C for a 6:30 bleach step that suggests that kinetics are probably not limiting. This issue of kinetics can actually get into some fairly complicate subtleties. My PhD research was on chemical kinetics, so I am aware of some of the potentially troubling subtleties that can some up in kinetics experiments, but I won't go into them here. (For the technically-inclined, this relates to whether it is a first or second order process in ferric ion concentration.)
On page 4-2 (table 4-5) it gives the process cycle for minilab processors, and it gives bleach time range of 4:15 to 6:30 at 100 F, which is another indicator that kinetics might not be limiting.
That leaves a possible issue of capacity. In single shot mode. I suspect that this would not be limiting, partly because of safety factors that the designers of the process probably built into the system, but there is another thing that might make the process very robust in this regard, and that is that there is a lot of agitation of the solution in a rotary tube processor. This would likely keep the bleach solution well-oxygenated while it is doing its work, which would tend to regenerate the depleted iron ions (Ferrous ions) back to the active (ferric) form. This is assuming that the kinetics of oxidation of the ferrous ions would be sufficiently fast to that it is not limiting and also that there would be sufficient oxygen in the tube so that it would not be greatly depleted by the ferrous --> ferric process.
Anyway, based on the comments so far I am at least somewhat encouraged about the possibility of using diluted bleach.
I do wish that my Photo-Therm processor would allow saving the bleach, but it does not, which is why I am investigating plan B to make the process economical, which is to use dilute bleach.
Also, I see that mshchem posted while I was writing this post, and koraks also posted another comment. Thanks for those comments, especially the comment by mshchem that bleach has huge capacity, which was based on a comment by Mick. Also the comment by koraks about having used dilute bleach in the past.