You can flush for 1 week straight and still pop positive. You cannot "Flush" an oil that is insoluble with water. Oil and water do not mix. THC Metabolites(which are oils and the shit the test is looking for) attaches itself to fat cells in your body. When you "Flush" all you are really doing is diluting the sample, if they are doing a full panel your specific gravity will come back all fucked and the normal amount of creatine will also be so low as to be undetectable, which then shows as a positive for dilution, which means you have to take it again, or in some rare cases is a failure by your employer.
Heat up 2 oz Synthetic urine to body temp and use the substitution method instead, works better than ANY thing other than your own CLEAN piss.