I don't exactly have a specific regime anymore. But I tend to do both full and partial changes. There are a whole lot of variables that affect my decision weather to do any change or not, but also when it's time to do any change. Typically I build the systems to do full changes, and mostly do. But I rely on observing the plants to tell me when to do what. Things like looking for changes in good color on the foliage, how much water the plants are drinking up (and being replaced in-between changes), odd pH swings, sturdy stock/stems, root color, etc. etc.. I also often replace the water the plants drink up with a diluted
nutrient solution periodically (depending on variables).