If your phototherm is a Super Sidekick, you can configure any of the chemicals to be output to the "save" port, not just fix. I run C41 in mine, and save the bleach for re-use. The other stuff I use one-shot since it's fairly cheap (Esp. fixer) and not too hard to come by.
A full tank on the super sidekick uses 480ml I believe, and can do 4 rolls of 120 or 4 rolls of 35mm or probably at least 4 sheets (or more?) of 4x5 film. They make a holder for 4x5 though I am not sure if you can cram 8 sheets into it or just 4.
While it's cool that Rollei or whomever is making a c-41 hobby kit, the price-per-roll is cheaper with Kodak chemistry or the 5L Tetenal liquid kit. I can get 44+ rolls of 120 out of 5L of chemistry, using the dev, fix, and stab. as one-shot and saving the bleach for regeneration later.
One of the things I love about my Wing-Lynch Pro 6, you can configure any (or all) solutions to reclaim with some simple plumbing. I run both C41 and b&w through mine, recovering bleach and b&w fix. The tanks hold close to 5 liters as well.
Back to original question: what happens if I dilute and extend the time?
Does anyone know?
I'm willing to bet some nasty color crossovers. End our passive curiosity, put us out of our misery, shoot a roll of Macbeth charts and have a go at it.
Just ordered the 20 roll kit for my shoots next week. I was a little frustrated that I can't get any C-41 locally except *maybe* Kodak (stock levels are questionable, and it looks like only big quantities are available), and B&H is now listing the Tetenal liquid as discontinued.