The reason I only recommend swishing for 3-4 minutes is to avoid
1. too any toxins, bacteria metals etc, being released at a time.
2. reabsorptions of what has been released.
15 minutes is a long time to hold the oil in the mouth. Some people can afford it, others cannot. I tend to be on the cautious side. If you want to do it longer than 3-4 minutes you may need spit out the oil, rinse the mouth and use new oil. I would avoid doing this in the evening, especially a longer version of it. The nervous system is very sensitive to toxins circulating when it is time to rest and sleep. Doing things faster doesn't always get you there faster.