I recommend a complete change of nutrients once a week to once every two weeks. Although some argue that this is a waste of nutrients and you can just top up the reservoir when the nutrient level has gone down due to evaporation and transpiration, this may be true.
However, depending on the plants nutritional needs, it may be taking up more of certain nutrients and less of other nutrients...topping up can lead to toxic nutrient levels with some nutrients and deficiencys in others. If you consistently top up with plain pH adjusted water, eventually, the nutrient solution will become depleted of nutrients and the plants will suffer. When nutrients are first mixed..the nutrition is very balanced and for this reason I recommend changing the buckets out at least once every two weeks and preferably once a week. Be sure to scrub the buckets out well with hot water each time you change the nutrients.