Typical mileage expectancy?

Anecdotal evidence strongly suggests EGR issues are a contributing factor, but nothing has been proven to be "the" cause. Most likely a chain of issues that ultimately cause the head gasket to breech.

 

Many have cleaned and even replaced egr components and still blew head gaskets. The best evidence is the 2010-2014s with poor piston and ring design are the chronic head gasket victims without the traditional overheating cause. Toyota acknowledges the piston and ring flaw which was revised mid 2014. This gen continued to have a poor pcv and egr design through 2015. Overall it was a very bad engine design that was completely reengineered for the 2016 models.

To understand the weak points, study the 2016 design where everything around the cylinders was revised.
Did 4th Gen fix 3rd Gen? | PriusChat

 

Perhaps they cleaned EGR at the last minute?

there’s another case reported here: car had head gasket replaced, and EGR still not cleaned, and blew that head gasket within 20k miles. And did the same thing, with same outcome, again.

It’s now on HG no 4, with cleaned EGR and intake, and so far so good.

Is there evidence that 2015’s aren’t blowing head gaskets?

 

I've been driving for about 60 years now.
And NEVER had a vehicle that wouldn't still provide transportation no matter how much rust the body had.
And I owned 2 VW buses and a Pinto in the mid-west so I've seen my share of rust.

 

Well, it is what it IS.
Better to shop at your convenience than to have the Prius "go up in smoke" and have a crisis on your hands.

 

Well, body rust is one thing, I'm talking about the under carriage.
I have had 1 truck with the frame almost rusted in 1/2.
1 car where the suspension a frame removed itself from the frame because of rust
1 car where the strap let the fuel tank go.
etc.

 

Great, I find this out after I buy the 2013. Is there anything we can do to prevent HG issues?

 

Periodic EGR and intake manifold cleaning.

 

That is a huge consolation, 280k is great!

 

The whole maintenance issues on 3rd gen are over blown imo. I wouldn't seek out a 3rd gen on purpose but if I took possession of one before I was aware of the issues (spoiler, I did) I wouldn't start reconsidering life as we know it.

It's still a good car and a lot of people have plenty of miles on them.

 

This.

So many who complain about poor design on them etc... need to go experience a KIA of the same or even newer vintage and re-evaluate their perspective.

 

What do you think using additives regularly like Gummout after cleaning Intake Manifold? and also pouring into the oil tank soon before an oil change for the sake of faulty rings of 2010-14?

 

No clue. The rings are not exactly "faulty"; they're lower tension, by design, to improve mpg. Unfortunately it seems they went too far in that direction, and different rings (and pistons) were swapped in, somewhere through model year 2014. All I can suggest is frequent oil changes, say 5K miles or 6 months max.