I don't think the mileage was ever mentioned, but you raise a good point -- it had to be stupendous, considering it was his work vehicle as well as personal transportation for all those years. He apparently drove it everywhere, including Mexico. Of course in real life, they had to get a "new" car when the ABC series started, and there were several along the way. But I believe that within the Columbo universe there was just one car, one raincoat, one suit and pair of shoes, and even one dog, no matter how impossible that seems. And whether Columbo ever returned to Mike's Garage, I would assume the Peugeot got as little maintenance as possible.
Except there was more than one pair of shoes. He ruined a pair in The Most Crucial Game and got a new pair, but knowing Columbo, he went back to the old pair after they dried out.