Corrected entry: Samara gives her victims seven days to live because that's how long she was trapped in the well before dying. The thing is, she had water (albeit not very fresh, but probably drinkable) so she would have lived close to a month before dying of starvation or, more likely, would have asphyxiated after a very short time because there was no air getting into the sealed well.
Shay
2nd Sep 2003
Correction: She probably died of cold or drowned, because water was too deep to sleep in. Combination of all the factors could easily kill in 7 days.