You can definitely get refeeding syndrome after 7-10 days of fasting. It depends on your your fat reserves, health, and lifestyle.
Say I'm 192cm and 69kg. My BMI is 18.7, which is definitely low, but perfectly healthy. I have an active lifestyle and burn 3500kcal per day. If I fast for 7 days without changing lifestyle, I lose roughly 7*3500/7700=3.2kg of bodyweight. By that point my BMI is 17.9. According to this paper[0] with 632 citations, anyone with a BMI <18.5 who's not eaten for more than 5 days is at high risk of refeeding syndrome.
If you think the numbers are fictional: those are my numbers from ~5 years ago.