Hi Esther! (Love your name!)
On average, most people lose around a pound a day when water fasting (it depends a bit on your BMR and daily activities) so in theory, 7 kg would take around 15-16 days. However, I lost less than a pound a day on average during my 30 day fast, and I’ve heard of others who have almost lost the double with a kg a day, so it depends on how much excess weight the body has to begin with, and how much water the body has been holding on to!