average miles for Front VS Rear brake pads and are they "screamers"?

They do (at least, earlier generations do, haven't looked at a Prime's) have the curled piece of metal that will touch the rotor and make noise if you have waited too long to change them.

I would not go nearly so far as to say what noise they'll make or for how long or whether they'll stop making the noise at some point or why on earth you would wait for that to happen when they are already there to indicate you've waited too long to change the pads.

Toyota's warranty and maintenance guides (at least for earlier generations, for sure) recommend a regular schedule of brake inspections, and I second that recommendation, because there are a bunch of things that can go wrong with the brakes besides the pads wearing out, and when you catch those things early they often cost you nothing to fix, and when you don't, bigger problems can develop faster than you expect.

So I inspect mine every time I rotate tires. That's actually more often than Toyota suggests, but hey, I have the wheels off anyway.

The limit for remaining pad depth that Toyota gives (again, I'm going by earlier-generation manuals) is 1 mm. At an inspection, if they're down to that, replacement is indicated.