so if/when C goes away, how long does toyota have to supply parts?

Something like 7 years was that timeline many moons ago when I worked for a company that supplied materials to Toyota.

If still the case, you’ve got some time and there are always junkyards for used OEM parts.

 

They're going to be making spare parts for these for a loooong time.

7 years by law if I remember correctly, but likely much more than that. Even though these were never popular in the USA, this car has never dropped out of the top 5 in Japanese sales.

 

Parts have to be available so dealers can make warranty repairs, including repairs under the emission control and hybrid system warranties, for which the minimum coverage periods are, in some cases, set by laws or regulations such as 42 U.S.C. § 7451(i).

At least in the U.S., I don’t know of another requirement that automakers make replacement parts available for a specific length of time after a vehicle is made or first sold, or indeed at all. I’ve seen articles and postings that claim various numbers of years, but never with a citation to a specific statute, regulation, or case; if anyone can point one out, I’d be grateful.

Clearly most automakers offer parts for out-of-warranty models as long as there is sufficient demand, and they may impose their own requirements on their suppliers to this end, but as far as I know, they choose to do this as a matter of good customer service—and to protect their brands’ reputations and the resale value of their products—not because of a legal obligation to do so.

Considering the world-wide popularity of the Prius c/Toyota Aqua, I expect most of the important parts will continue to be available, both from Toyota and aftermarket suppliers, for many years.