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.
Situations where notice is not required. An employer is entitled to at least one week's notice from an employee who has been working for the employer for 13 weeks or more. If an employee has been working for an employer for less than 13 weeks and has no contractual obligation to give notice then the employee is not obliged to give notice.