Fast growing crops consume large amounts of nutrients and water so it is important to keep an eye on the reservoir every couple of days. Also consider the size of the plants(bigger plants, more water consumption) and how warm and dry the environment.(hotter, more transpiration).
Yes. The reservoir should be drained and rinsed every 7-14 days, depending upon plant size and nutrient usage. It should definitely be drained and cleaned whenever you change the nutrient ratio formula. It is also a good idea to rinse off the growing medium (expanded clay pebbles) each time the reservoir is cleaned. Plant waste will tend to accumulate on the growing medium.