Here are my thoughts, in a short form, of a biochemically trained brain.
Pretty much everyone here is familiar with the detoxing benefits of an adequate water intake.
Chemicals (molecules) come in two forms. Ionic and nonionic. Ionic chemicals are water soluble. When we detox our bodies, we drink lots of water to flush any unwanted toxins out of our body. WELL guess what? That's only half the picture. Nonionic molecules are soluble in body fat and cell membranes. *
By oil swishing, you are doing the same thing for your body as when you drink a sufficient amount of water daily. You are giving toxins a medium in which to dissolve, then by spitting the oil out, you are removing whatever toxins came in contact with and dissolved into the oil. Using OIL is beneficial because it dissolves a different chemical class of pollutant/toxin than water. This is why you see healing first in the mouth, then later in different parts of the body. The toxins must migrate to the mouth before they can be removed with this method.
This is also why switching to a different type of oil will create a new cycle of healing/detoxing. After you've oil-pulled long enough to "stabilize" your toxin level, you've removed as much toxin as you can with that particular oil. By using different oils with slightly different makeups, you will be able to dissolve slightly different toxins more effeciently. It is not a matter of your body getting "used" to anything -- it has to do with the chemical qualities of the oil itself.
With this knowledge, there are a few more things you can do to keep the good feeling going. One I'll endorse is to eat low on the food chain. Any time you eat fat from an animal, you're ingesting the fat-soluble pollutants that were stored in the animal's body. Guess who buys organic milk and butter?? Ha ha.
If you had an ample supply of appropriate oil, you could 1. coat your skin to excess, then remove after 15 minutes, or 2. use in an enema, preferably followed by a SWF (Salt-Water-Flush) to keep the toxins from migrating back into the bowel from any oil left after evacuating. These should have similar toxin-removing effects on your body, although they would be considerably more involved, messy, and costly due to the amount of oil required.
I hope this has helped some in the community understand the Science behind oil-pulling. It's not magic, and it's not even that wacky. It's a good, healthy, direct application of chemistry.
With this information in mind, you can see where oil-pulling for longer periods will give you a certain increase in benefits, but the daily wait in between cleansing is helpful to the process too. After all, the oil can not remove a toxin located in the bowel. The bloodstream and normal (slow) diffusion will have to carry it to the mouth before you can be rid of it. Think of your body as a sponge -- the oil can only clean one little spot of it at a time. Eventually the whole sponge gets clean... but it takes time and consistency.
Please excuse my spelling. It never was very good. ;)
* Membranes are made up of molecules with an ionic side (faces the water-rich world and the cell's interior) and a nonionic side (sandwiched together, creates a chemical barrier).