Until they are at the point where they can stand up by themselves and walk out of the room.
Seriously, every time - maybe will wear the same pair if I'm doing a double, so 1 pair per day would be an accurate answer. I've got a drawer that's overflowing w/ them, though. You can often get nice tech sox on sale at race expos.
A couple other points I learned from a clothing designer at Brooks:
Putting running clothing in the dryer, as long as it's not super high heat, actually rejuvenates the elasticity in Lycra.
Don't use Bounce or other fabric softeners in the wash or dryer - it will only mask odors, and actually lock in the odor-producing bacteria by clogging the spaces between fabric fibers.
For washing, don't use a detergent w/ a lot of perfumes or surfectants. Something like WIN, Penguin Wash, etc. are better for tech running gear.