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.
However, two weeks notice is not legally required. If you have a union agreement or employment contract that states how much notice you have to give, definitely follow those rules. Otherwise, do your best to give two weeks notice. This is a good way to maintain a positive relationship with your employer.