[a_imho]

From a very simple calorimetric pov 1 lbs fat ~ 4000/3500 cal, assuming using 2000 calories for normal daily activity one needs to spend an extra 2000 cal per day for 10 days, which is like running 20 mile with no energy intake whatsoever every day (for 3 days per week it is 40 miles for days when one exercises).

[Scaevolus]

Most of the weight loss would be water weight from glycogen stores being depleted, not fat being burned.

[Xdafjgneoiwe]

This is correct. You can expect maybe a kilogram of fat (2 lbs) per week on a very restricted diet. Much more than that is hard to maintain over a longer period and unrealistic. When you first start with a keto diet, you can easily expect 3-4 kilogram of water weight being lost.

[a_imho]

Exactly, which puts things into perspective regarding claims/opinions/recommendations.