It seems that pretty much as soon as the oil becomes thin it should be expelled. A person could time their swishing to get an idea of how long it is taking to get to that point. Such information may even be used to gauge how efficiently their body is detoxing.
I tried OP several months back, and have started again today with the intention of continuing and observing certain physical responses. I personally feel that 15 minutes is too long for me as I begin to struggle with the volume and thinness of the liquid well before then. But, I didn't have any toxic reactions all the same. I believe that most health protocols are most effective when tailored to the individual.
Perhaps people can report some of their noticeable results, including their herx responses along with the length of time they tend to swish. Perhaps over a larger group we can see if there is a correlation between time, adverse reactions, and healing responses.