I suggest dumping the nutrients every week or letting them go 1.5 weeks at the most before changing the nutrient solution. Especially during periods of intense growth. I sometimes let the nutes go for 2 weeks during the early stages when the plants are small and not growing at an insane pace. The reason is because the plants are using different nutrients at different rates so your nutrient solution will become out of balance. The root exudates will also modify the pH. Plants using different amounts of ammonium, nitrate, phosphate, potassium, Calcium, Magnesium, etc will also change the pH. When growing a high value crop, it is best to always be on the safe side and give fresh nutrients as often as possible. I mean, after spending so much $$$ on seeds/clones, lights, fans, electricity, etc... a 55 gallon res change that costs less than $12 (crazy growth, 10ml+10ml+10ml x 55 gallons at an average of about half a cent per ml which includes average cost of shipping) is a no brainer to me. Just my 2 cents