It's not about how long the oil lasts. All the adults are killed by the oil as they breath only though their skin, so they suffocated. You need to retreat when the eggs hatch out before those mites get to egg laying age to break the cycle.
That is the reason to add something that will kill the eggs too in my mind. See instead of doing this again and again for 8 weeks you can quit sooner. It's best to be sure, so do them a couple days in a row, then once a week twice. That way if you missed a spot it will not make you start over by reinfesting. That's also why I like to be sure the tea tree is about 5% so I tend to track down the ointment, even though it's more work and messy it seems. But you can sue the oil and mix it yourself, just do the math.