I wash my Smart Wool base layers every time I wear them. While they don't *stink*, they do have an odor. I have a front loader, put them in a mesh bag, in the regular cycle and hang to dry. My Smart Wool merino light 1/4 zips get worn many times, though between washings.
I sweat a lot, and while I don't necessarily have an odor problem, the thought of wearing a base layer without washing, well, doesn't set well with me. Now, if I was camping or something, I would have no issue wearing them for a couple of days.