when to replace coolant on 100k prius C

100,000 miles, or 10 years, is the recommended first change for engine coolant. Every 50,000 miles, or 5 years, thereafter.

 

follow your maintenance schedule for all service, ignore dealer advice and consider changing the tranny fluid

 

Don't forget about the inverter coolant; separate cooling system - since you weren't clear which one we're talking about. When you look under the hood, there's two pink coolant tanks. If memory serves me; the one by the hood latch is the inverter coolant and the one on the passenger side, internal fender, is the engine coolant. You can do a litmus test to see if it actually needs changing x2.
I'd do both of them at the same time to avoid the extra downtime.

Hope this helps...