If it's a guitar that's never been oiled before, and that has likely not been oiled for many years, it will soak up the lemon oil very quickly, certainly in less than five minutes. First I oil and then rub the board with a paper towell or synthetic sponge scrubby material to clean off all accumulated finger gunk. Then I let it absorb and oil again. I'll keep doing this as long as the board keeps sucking it up. When it takes 5-10 minutes to absorb the oil, I'm done oiling. Then I put the old strings back on and play it. I do this because, in my experience, the newly oiled board with cause the strings to blacken at first. Then, a day or two later, I restring with new strings.
Bill, tgo