Do I need to change solution in a DWC system?

[trinidadPepper]

Oh that set up in the pic is not actually mine. I borrowed that from someone on here.
Mine is home built actually.

 

[Schonke]

Late answer, but might still be relevant. I've got a couple of DWC buckets (2-5 gallon each) and I only change water every 3 weeks. I fill up the reservoirs every 4-7 days and correct ph every 7 days as well as adding some nutes to get EC up to where I want it. Been working fine so far, though if you do it this way you need to be aware of any nutrient deficiencies/lockouts and salt buildups that might occur.