I think flushing for 2 weeks is optimal, I had to harvest without a flush before, and the quality, taste, etc. suffered tremendously for it. You might be able to gain a little girth by using nutes, but it will affect the quality of your entire crop.
I do the following......ten day flush hydro, fourteen days if organic feeding in soil/soiless mix, or three weeks if using chemical fert's in soil/soiless mix, water & dark sugers during flush no food, last 5 days plain water only
10 days and use some thing with carb ie molasses,liquid carbo-load,and even sweet will work i use for 5 days and then floush with plain ro for the final 5 days