It depends on the plant, and whether it's inside or outside (and in what weather). Some plants need a little water daily, some twice a week, some weekly and some even less than that. The biggest...
If plants are not watered, the chlorophyll cannot be produced. Therefore, the plant cannot produce energy, and will eventually wither from hunger and die.
That entirely depends on the plant. Some plants have adapted to grow in wetlands and need to have their roots constantly submerged in water. Succulents (cacti) may prefer to go weeks without water....