What are the general feeding & watering guidelines for nft systems?

The rule of thumb is to replace the nutrient solution at minimum every two weeks. But do it more frequently if you have a large amount of plants relative to a small stock tank. pH can rise dramatically when using a small tank for a large amount of plants. So always keep an eye on the pH and adjust every 2-3 days to keep in the optimum range for whatever plant you are growing. A good guide for changing the solution is a high EC.

Also soft stems or less than perky green leaves would be an indication to change the nutrient solution. How often you should water depends on many factors. Best guide is to water when the GRODAN cube feels like it is 50% dry. Download the brochure "Watering and pH" for more information.