The average (perhaps geometrically, even) over all people is, as a statistic I've pulled out of a perhaps all too apt a place, about once a day, I reckon. At the extremes, there are those who go multiple times a day and there are those who somehow manage to last a week or more without needing to.
Extreme cases without diagnosed illnesses really ought to be looking at their diet IMO, but then, I'm no more an MD than I am a statistician.