Medical experts usually say the average person can live for 4 to 6 weeks without food. The amount of time someone can go without water is much shorter, roughly a week (or less).
Your employer can force you to take leave at a particular time as long as they give you at least two weeks' notice. However, it is common for employer and employee to come to an agreement about the timing of annual leave. If you put in a request for leave, your employer has to have a good reason to refuse it.
Generally, yes. An employee must tell the employer at least 30 days before taking FMLA leave if the leave is foreseeable (such as birth or adoption of a child, a planned medical procedure, or planned treatment for a serious health condition).