Experienced fasting supervisors, routinely note that even on water, the larger gains are not achieved until the second week, and that for people trying to get resolution of illnesses, days 15-21 yield the greatest results.
It appears to me that Burroughs saw adequate improvement in most people in 10 days, and so he suggested that as a minimum because in his experience anything less lacked the efficacy to do what he was looking for. However, that still doesn't mean those with problems could not ramp up over several different fasts to the 10 days, especially highly toxic persons. I've never had any trouble with the 10 days, and I'd still recommend a full day and night Water Fast to those who are, so they understand how easy the lemonade is. With no fasting experience there is no point of comparison. One full day and night on water ought to bring perspective via a bit of suffering, although when I get through the first day on water I feel the accomplishment warrants continuing the additional 1-2 days on water after which euphoria typically replaces "hunger pangs"--it's not honest to goodness hunger, but the body/dumb flesh, rebelling against not being gratified, grumble, grumble, "Feed me, Feed me", you remember the movie :).