I realize the math is never as straightforward on paper as it actually works out in your body, but assume someone's TDEE (total daily energy expenditure) is lets say 2,500 calories. 1 Lb of body fat is roughly 3,500 calories. This means that a 24 hour fast would only utilize .71 lbs of body fat (again, this is assuming the ideal condition where you are RQ =.70 100% of the time burning entirely fat instead of a combo of fat/carbs from glycogen stores which is much more realistic).

"Lean" is a fairly general term, but I think it would be agreeable that a male with 10% BF would be considered "lean", so I will use that. Let's say said male is 6', 185lbs, 10% BF. This means that 10% or 185lb *0.10 = 18.5lbs of their weight is body fat. at .71 lbs of fat/day, that would be 26 days worth of 2,500 cal burn/day (again, ideal scenario only burning fat). Of course this would mean 0% bodyfat which is impossible; but even if they were to "fast" down to 5%, that would be 1/2 that or 13 days.

Until you are pretty low (in the unhealthly range, such as less than 5%) of body fat %, then multiple days fasts are practical and possible.

...Especially when approached intelligently and in a healthy manner.