2012 Prius when to drain or flush coolant?

Check the maintainence book. It will tell you.
But now is a good time to replace the engine coolant and inverter coolant.
Only use Toyota fluid. Can't go wrong with that. Mixing coolant is not good.

Spark plugs are due. The coils are fine. 335,000 miles on mine.
Unless you have an issue with them.

Are your brake pads worn out? Or are you just replacing them, because?
120,000 miles is not alot for Prius pads, unless you are hard on the brakes.

 

Toyota USA says 100K miles or 10 years* for the first engine coolant change, and 150K miles or 15 years* for the first inverter coolant change. FWIW, Toyota Canada says 160K kms (~100K miles) or 10 years* for both. After the initial change, it's 50K miles or 5 years* thereafter, for both.

* whichever comes first, miles or years

I did a summary of the event-by-event schedule in the Toyota USA Warranty and Maintenance Booklet, attached:

 

" A lot of people said..." Be careful who you listen to. A lot of people are wrong

Why are you replacing the brake pads?

 

If your using OEM Toyota coolant, a simple drain and fill is good enough. Do both inverter and engine coolant at the same time to save downtime.
Dealers and mechanics will try to 'sell' you a flush, for more money. The only time that's needed is if your changing away from OEM coolant or you've got contaminates in your cooling system. Sometimes they just do a drain n fill, then call it a flush. I've seen that happen.
That's why the OEM manual states 5 years or 50K mile there-after the initial change. The engineers know your not getting all the coolant out of it. Some old residual mixing with the new stuff. That's the other issue with a flush - unless your using concentrated coolant, you won't be able to get the concentrations correct - unless your using a flush machine. The flush machine will also mix new and old coolant.

Just my 2 cents....

 

Maybe “flush” is in fact synonymous with drain and fill? Seems like.

 

If someone was telling me that they were going to 'flush' my cooling system - I expect them to flush it. It's not the same as a drain n fill. Ditto, when it comes to brake fluid. Sucking the brake fluid out of the reservoir and replacing it, can be considered a change - but definitely not a flush.

Does My Vehicle Need a Coolant Flush? — Tommy's Hi Tech Auto Repair