I found nothing about the frequency of washing.
I did find this:
You can't simply add products like Oxi to your washing and expect miracles. OxiClean (Borax, Amaze, and Arm & Hammer Washing Soda) are laundry boosters, which means that they'll help to brighten-up average laundry. Boosters can also be used as stain removers, but they have to be applied directly to the problem area and allowed some time to work. (This is true of all stains, not just a shirt collar.)
Most people agree some collar (and the cuffs of shirts) staining is due to sweat, which is true, but only partially so. Collar darkening is caused by a combination of sweat, dead skin cells, skin oils, and soap product buildup. Sweat can't be helped, but if the collar is a persistent problem for you, try to pay a little extra attention to your neck right at the end of your shower. Doing so will help to remove dead skin, as well as any residue shampoo, conditioner and/or soap you may have left behind. If you use hair products, you may also want to give your neck a quick wiping off after you've done your hair but before you put on your shirt.
Directly apply your preferred laundry washing product to the problem area, but be ready for the possibility that it might not work as well as you want. DO NOT give up if that happens. Try a different product. This is sort of a frustrating thing to have to say, but unfortunately the truth is what works on one person's stained shirt may not work on another person's stained shirt. There are a lot of different possible combinations. One shirt was saved with OxiClean's spray stain treatment on the collar and then rubbing the collar against itself before washing so that the fabric absorbs the detergent. Another person tried the same thing and pfft. Nothing. A different kind of product he tried did work.
Sprays: Spray stain treatments are probably the easiest of the bunch to apply. Just spritz spritz spritz, and then work the product into the stained area by rubbing the shirt's material against itself. You can also use an old toothbrush or a laundry brush to help work the product in. In the spray category, some brands to look out for are:
- Resolve/was sold under the name Spray 'n Wash
- Charlie’s Soap
Bars: Using a laundry bar will take a bit more effort than a spray—though really only a bit more. The upside is that the force of rubbing the bar into your stained collar will obviate the need to rub the shirt's fabric against itself. In the laundry bar category, some brands to look out for are:
- Fels Naptha
- Dr. Bronner's
Gels etc.: Products that you need to spread onto a stain using your hands:
- Mom’s Goop
- Liquid laundry detergent
- Dish soap
When in doubt, reach for the dish soap when you don't know what it is or where it came from.
Other kinds of soap will work too—liquid laundry detergent, hand soap, bar soap—but dish soap is one of those things that's almost always on hand and is also my go-to whenever anything gets spilled in my house. Splashed some coffee on the duvet? Dish soap. Dropped a piece of dressed lettuce on your pants? Dish soap. Have a mystery stain? Dish soap.
The thing is about dish soap is that it's mild, so the worst thing that will happen is that it doesn't work. Usually, with a small amount of dish soap and a sponge, the problem will disappear.
If the worst happens and the dish soap doesn't remove the stain, head for one of your stain pretreatment products. Again, the use of a stain treatment isn't going to hurt the situation, though of course it may not help.
Nylon (specifically) can be machine washed using either cold or warm—but not hot—water. It's best to air dry nylon, though it can be dried in the machine using a no-heat cycle. Unfortunately, most nylon items cannot be dry-cleaned because the solvents will melt the fabric, but in the case of mystery stains on clothes that can be dry cleaned, sometimes the best course of action is just to throw a few bucks at the problem and let the professionals figure it out.