Two weeks is the norm for flushing... The yellowing is from a lack of N. Just because it does not turn yellow that does not mean anything.
Also like many people think that when you stop using nuts the plants use up all the stored, this is not true. the plant can not absorbe all the nutes that are stored in buds/leaves/roots so you can flush for 2 months but you WILL still have nuts left in the plant, this is the reason MOST people flush for about 2 weeks because anymore is "kinda" but not completely pointless and anymore and you usually have funky tasting bud, this also depends on your ferts. Also MG nutes DO NOT leave a harsh taste, that is just when people use WAYYYY to much during the plants entire life than tries to flush everything and the plant can just not do that this is the reason for the funky taste it has nothing to do with the actually fertilzer it self.