If you ask 10 people, the "flushing" question you will get 10 different answers so I think there is a better question and that is what system are you growing in? 
If you're in a soil or soilless media growing in pots then a salt buildup is a potential issue during the entire crop and flushing weekly helps alleviate this problem. If your crop is near the end just prior to harvest, then then you might just want to do a sequence of three things. 

1. Test the TDS or EC of some runoff water coming out of your pots to see what remains in the soil and is not being used by the plants. If it is a higher number, then I would suggest flushing a couple of times and if it is a reasonable or lower EC number the I would suggest flushing on the last two days.
2.  The goal is for the plant to use up all nutrients and to be 0 by the end and of course that is impossible but it gives you a target.
3.  Pick the day you stop feeding entirely and then do nothing for a day or two followed by a flush of straight water or a fulvic acid and water mix (1-2ml/L) and flush to the point of water leaching  (running) out the bottom. This should grab all the remaining nutrients and flush them out.