I change my nutes out once a week, making a new batch each time. It isn't technically waste however since I use the water for my house plants haha! Even with spent nute solution they are thriving really well. It tells me that GH really doesn't care about making a million since their product is so strong you can't even think about using it full strength!
I use the 3 part system and not the Lucas formula. I haven't ever used that regimen so I can't comment on it. It must work though since people are really happy with it! Also, since I have no actual budding sites yet I am nowhere near flushing. To tell you the truth my last plant (Green O Matic) I didn't even flush...
Just consider using the harvest method that I wrote about since the timing with autos in DWC is so forgiving (you don't have to worry about the whole plant dieing off at once). This is the reason I didn't go for the flush last time since I intentionally let my Sugar Mango Ryder flush for weeks and nothing happened except the buds got over-ripe and produce an effect similar to Nyquil haha!