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.
Notice you must give your employer. If you have worked for your employer for one month or more, the legal minimum amount of notice you must give is one week. Normally your employment contract will set out a longer notice period. If it does, you should give this length of notice to your employer. If your employment contract does not set out a notice period you should give a reasonable period of notice to your employer. This is included in your employment contract as what is known as an 'implied term'.