As many times as needed per day to keep your feet from smelling. This may be once a day, it may be three times a day. All depends on your physical activity.
Fresh pair in the morning and I try to change them around mid day if possible. It's just a nice feeling having fresh feet through the whole day and I find one pair doesn't really cut it weather I'm being really active or not.
What a weird question. Depends on how easy your feet sweat, temperature, type of shoe etc.
For example, in winter, if your feet sweat very little, plus they don't sweat almost at all because it's cold, and you sit in house with no shoes, you could wear a pair for a week since... they don't get to dirty.
However, if you are in summer, you sweat easy and you wear sports shoes that don't allow air to circulate, you will need to change them every time you remove your feet from the shoe... because they're dirty and they smell.
I admit, upon reading the title, I really thought it would go like this.... how often should someone change their underwear.
Weird topics of the internet <3