I am sorry to read about your grandmother. As for how long that can vary by religion, state laws, or when the family feels every one can attend a funeral In most cases it is somewhere from 3 days to a week.
The funeral should take place at least a week and a half after the death of a senior. This should give the family enough time to arrange for the burial as well as clear all pending medical bills if any. However, this can vary depending on other plans that the family has.