i once went on a diet of about 1000 calories a day and after about the 3rd week i started gaining weight (ended up weighing about 7 pounds more then when i started). of corse everyone is different, my body type is probably drastically different then yours and the type of food i was eating (hign in fat low in nutrition) probably contributed to the eventual gain in weight, and the fact i wasnt exercising. theres no general number of days or calories that trigger the change because
everyone is so different, and there are probably differences in how much people gain. one person may gain 10 pounds in a month some people 3, some people it wont effect enough to make them gain any. just like theres no set date for getting bald or getting your growth spurt. biology is inconvenient that way
but it is a fact that its real, despite alot of peoples opinions that since they didnt gain weight while starving it must not exist (normally i dont want to be a poo but when it comes to matters of health its important to know facts)
im real lazy and dont feel like researching it more then i have previously but heres a link that cites a bunch of studies n stuff.