It's always better to be safe than sorry, even if that means buying shoes more often than one would like. It's not unreasonable to figure that you should replace running shoes that get regular use once every six months or so. Particularly if you're a moderately heavy runner, or if you run on hills a lot, which I bet you do in Portland. (Ever run up in the West Hills? I used to run there when I trained for marathons.)
It's worth noting that the 804s are a trail-running shoe. Nothing wrong with that if that's what you're doing. But trail runners typically are a little stiffer than road runners, with a higher density (i.e., harder) midsole. So they don't cushion quite as well. And all that impactprobably 500 to 600 pounds per stride for a guy your sizehas to go somewhere. If the shoes don't soak it up, it goes right up into your knees, not good for the cartilage. So if you don't have stability problems, go with a shoe that has more cushioning. The Asics Gel Nimbus IV ($100) is a shoe with lots of heel cushioning. The New Balance 1022 ($100) is another high-cushion shoe. Incidentally, I think NB shoes are among the best out there.
New Balance 1022s available here; Asics Gel Nimbus' here.
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