New Bike shoes

In my experience, bike shoes can last a long long long time. I'd only replace them if the uppers were truly worn out from sweat/torn/etc or if something broke on a buckle/strap. Otherwise its maybe replace the cleats on the bottom when they are worn, but thats a different (and much cheaper) story. So to answer you, no I don't change them every few hundred miles, or even few thousand miles.

the bike shoes I just stopped using (and they are still usable) are 8 years old and over 50 races on them and 35000 miles plus on them. one of the buckle is a little tricky to close now so I could not race with them anymore.

makes me wonder how much more time I have on my bike frame it has about 28000 miles on it

Odor

My family refused to ride in truck with my shoes in the cab. The shoes got soaked in a rain storm after a HIM and were never the same again. Great shoes and no issues other than the odor. I had them about five years of riding and racing before I purchased new shoes.

I get new bike shoes when they test 12w faster than my prior ones

See my testing: http://www.thomasgerlach.com/2016/05/fastest-ironman-bike-shoes-aer...

Smell

Though the last pair was trashed after the buckle was ripped off in a crash.

They last a long time so go ahead and invest in a nice pair. But you've got to replace the cleats from time to time.

To stop the odor I use a boot warmer after every ride. Drop shoes on and let them dry out over night. Shoes are four years old and don't smell a bit.

Yep, if kept clean, they'll last 10s of thousands of miles . . . like others said, new cleats every once in a while depending on how much they're walked on