Feet produce more sweat per square centimetre than almost anywhere else on the body, and socks absorb all of it.
The short answer
every wear
When to wash sooner
Wash it now if
- they have been worn
- no exceptions
What to do between washes
None. Wool socks are the one partial exception and even they only stretch to two wears in cool weather.
How to wash them
Warm, with like colours. Wash wool socks on the wool cycle or they felt and stop fitting.