When to change solution for hydroponics

Definitely every week. I like to give a fresh batch every 7 days. If you leave the nutrient solution too long the plant could have depleted certain nutrients out of the solution and a deficiency could start as a result. Happy growing

You can refill and re-establish the right levels in your reservoir throughout the growth cycle even if it is highly recommended to proceed with the change of the solution at least once every two weeks in the optimal conditions, once a week with tap water would be better as HarbingerOf Harvests has well said

If you use RO water, A closed loop system and water chiller. you can go longer.. But if your using tap water and don't have a chiller change weekly.

every week.. it's more so for cleanliness and to top the TDS level and Accurate.. after awhile your nutrients will be contaminated by the air.. Do weekly Changes.. it wastes water but your plants will thank you.