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.
The total period, with the extension, can't be more than 24 months. An employee has to give written notice if they want to extend their leave. It has to be given at least 4 weeks before their original leave ends and state the new end date.