Flushing depends on growing method and fertigation style. If using this line in a bubble bucket or media-less culture, there is no need for a flush. If this is used in inert media such as Coco Coir or Rockwool flushing is determined by continuous liquid feed concentration, environmental condition, evaporation, and other fertigation and environment factors. 

Good rule of thumb is in Coco an RO/Microbes flush is beneficial during weeks 3-7 periodically (between 2-3 feedings) to ensure balanced rootzone chemistry. The fertigation program will determine if this is needed but as most growers seek to maximize their plant growth higher EC programs are more likely used. This higher EC use tendency increases the need for periodic flushes of the growing media.