I'd just wash it when it seems like it's dirty, or around once a month, not replace it, unless that's the one you're using while the other one's in the wash, then you wash your spare after the allotted time.
About every week. You son't want to waste water and detergent but you don't want to live in a dirty environment either (eg. Lice) maybe you have a different opinion but I think that people should change 'em once a week, cause that seems just right!
Once a week is fine for washing your bedding. If a person is suffering from acne then they may wish to wash their pillow case two times per week.
How often should you change the fuel filter?
as often as needed
You should change your oil every 3,000 miles. If you drive your car rough, then you should change it more often.
Silk pillowcases would be for sale anywhere that linens are sold. Bed Bath and Beyond, Macy's, Sears, and Nordstroms are all good places to locate silk pillowcases for sale.
There are a number of options for one to find and purchase satin pillowcases. These are available from Sears, Kmart and Bed, Bath and Beyond. Satin pillowcases can also be found online on Amazon, Overstock and eBay.
everyday
There are several places that offer silk pillowcases. You can try Walmart, Sears, Belk, Nordstrom, Macys, or Kohl's. They all offer silk bedding, including single and double packs of silk pillowcases.
How often should you change water pump in a boat
I will donate pillowcases to a US nonprofit which is sewing dresses for Ugandan orphans!
Every week
You should change them after every feeding because they should be washed after every feeding.