I do what Sanctuary does in that I rarely keep the same oil in my mouth for the fill 20 minutes -- I usually go through two or even three mouthfuls in and OP session and total 20 minutes or more.
What might be helpful to understand is the theory behind the 20 minutes' prescription -- that it takes that long for the blood to circulate throughout the body via the sublingual veins, so by OP for twenty minutes, you are cleaning all off the blood.
That said, I don't worry too much if I miss a few seconds or so -- I feel like I'll get that blood the next time! But if I have to stop for a conversation, etc. I will start all over again later so I can reach a total of close to 20 minutes, at least.
It's hard for me to accomplish that in the morning before eating as we are supposed to, so I usually wait until nighttime. And I do think it will get easier to go for more than 5 minutes with time and practice....
Good luck and happy pulling!