Flushing Question

I have been doing research on flushing before harvest and every thing I read says to flush 2x the size of your grow container ex.5 gal=10 gal. My question is should the 10 gal flush be done on every watering till harvest or should the 10 gal total be done within the last 1-2 weeks before harvest?

Just once to flush out any excess nutes.