I have mid-weight Smartwool that I wear for commutes from now until mid-March or so. I might wash it every two weeks (usually 6-8 commute days in the winter, 22mi RT) but never because it smells, just because I feel like I should.

I wash it on gentle cycle/gentle spin in my front loader with other bike gear and lay flat (or occasionally hang on the bike) to dry. I also have Smartwool socks that I wear year round and the oldest pairs are now three years old and they're not really showing much wear (I do wash those after every day or two).