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.
It's common wisdom that when you leave a job, you need to give at least two weeks' notice to your boss. That's definitely a good general rule. After all, if you leave without giving two weeks' notice, you run the risk of damaging the relationships you've built with people at your workplace. Even if you don't like your boss, giving less than two weeks' notice can place a huge burden on your co-workers. Professionally, not giving your two weeks' notice before leaving a job can hurt you in the future. But legally, you don't necessarily need to do it.