MG is shit.... dont ever use it again. spend a few bucks and buy 4-5 bottles of actual organic nutes... then you wont need to flush. even if you use organic MG you still will want to flush and believe me theres a good chance you will still taste that shit.
I would flush 2 weeks b4 finish, depending on how much of it you used. if you used alot i would flush again prior 4 weeks b4 finish. b4 you start make a quick screen to stick in the drain so you dont clog it with dirt. take each pot into the bathtub, however big your pot is run 3x that size amount of water thru it... let them sit for 45 mins or so to drain then back into your room.
dont water them again after that. its best if you almost let your plants dry up and die right b4 you harvest.. this gets all those last nutes into the buds. the plant knows its main mission is the fruit so every last resource it has will go into them.
I personally use 1/2 promix (or a soiless medium) 1/2 perlite. excellent drainage, stable base and easy to maintain proper PH. my fav nutes are called Earth Juice and totally organic