I don't get very many nags at all. However, I have noticed that it nags at the same spot on the road every time, regardless of my speed. There was one drive where it seemed to be timed. I watched it. But that has not happened since. Sometimes I can go a good 30 minutes before anything. I would guess, on average, it is probably around every 10-15 minutes when I am on a longer drive.
I find it odd that it seems to be so different for so many drivers and I wonder if speed plays a part in its calculation. I have clearly seen how uncertainty does play a role. When she gets nervous, she nags.