The question is: how much muscle can you stand to lose?
There's a religious nutcase who's posted a series of videos and follow-ups to her 3 consecutive 40 day water fasts. The were separated by about 8 days of eating. Water fast means: she was drinking water, and taking a multivitamin, but otherwise no sustenance. She dropped from 200# to 100#.
On a reduced-calorie diet (or straight starvation as in this case), about 20-25% of your weight loss is lean mass (muscle and organ tissue). This woman lost a maximum, then, of 80# of fat, over 19 weeks. That's 4.2# per week.
You can sort out what your own baseline metabolic expenditure is with a basal metabolic rate calculator. If you're big enough, that gets to be considerable. My current maintenance calories are around 4k/day. Pack on another 200# of bodyweight (450#) and that blows up to 4200 cal/day, using a standard estimate. At that rate, on a starvation diet, I could be burning through as much as a pound and a half of fat per day. At least by the numbers.
But I'd be catabolizing a lot of protein as well.
I figure 1-2# of bodyfat per week, or 1-2% of body weight in fat loss (for a 400# person, up to about 8# per week).
But for most of us, 1-2# makes for a really good planning basis. Back about 4 years ago when I dropped 50# in 9 months, the average rate was 1.3# per week, nearly in the middle of the predicted range. Managed to pack on some muscle while doing it too.
At the lower rate, you've still got enough energy left over to train and build muscle. If you cut carbs and fats but leave protein (a protein-sparing modified fast), you've got energy (from diet and stored fat reserves) to fuel catabolism, but your overall energy balance leads to fat loss. The strength training triggers anabolism (or at least holding on to muscle mass). Actual results depend a lot on where you're starting from (my results above were from a pretty de-trained state).