How long can C-Biscuit sit without being driven?

you might want to squirt some gas saver in the tank and drive it once a week.

the 12v should have a smart charger, or disconnect the neg terminal.

watch the bars on the hybrid battery, i would drive it enough to keep them at 4 or more

 

No special sensitivity to fuel.

I think bisco is referring to fuel stabilizer, to prevent chemical breakdown. A 2oz bottle sta-bil is a fine investment.

I'd say once a month is fine for exercise.

 

You're over thinking it.
The "C" engine is basically multi-fuel Atkinson cycle [High compression ratio but the cylinder pressure is bled-off by cam timing]
The O2 sensors and air flow meters will correct this.

I have a Ferrari that was last gassed up in February 2022 and sits on a battery tender. When I started it, I could smell the Ethanol initially then it goes away. [So I don't give a sh**]

Ethanol will not evaporate in the fuel lines and fuel tank which is a sealed system, and any fuel returning to the tank mixes it back up.
It is more off a problem on carburetor engines.

 

I have parked my 2012 Prius-C in my garage since May 2000. During the past 2.5 years, I started (not drived) it only every 2 - 4 months to charge the traction battery, and never see battery level bar move before charging it.

I also add fuel-stabilizer to the fuel tank. Last December I sipped out the fuel from my Prius-C and put it into my Prius. My Prius performed normally with the 19 months old gasoline.

The auxiliary battery needs more care. I disconnected it from the car and charged it once per month.

I also partially lifted the car on 4 jack stands to prevent tire flatting.

 

I have left my C parked for 6 weeks without doing any special treatment. Just hopped back into it and turned it on.