Well I don't wash my turtleneck after every wear but maybe after 3-4 wears? I wear an undershirt usually but the neck part still gets oily after a few wears.There is much talk among the new wool shirt brands that this sort of thing doesn't need laundering the way cotton polos do. Just the same, I have been looking for long-sleeve t shirts (V neck or Henley) to go under my new winter polos. The virtue of being casual and not sending a dress shirt to the cleaners gets pretty well negated if you have to hand wash and air dry wool or cashmere every day.
For crewnecks and cardigans I wear on top of shirts, I wash them at the end of the season unless I spill anything on them.