That's an old myth. It takes four weeks for fresh brain cells, produced in the brain stem, to "travel" to the grey matter cortex of the brain, where they take over active functionality. Scientifically proved by english students about a decade ago, where they put a heavy alcoholic on a "diet" of 1 shot per day. After a month, an increase of intelligence and brain mass (60% !!!) was measured.
Also, two and four weeks are typical timespans to automate and integrate newly trained thinking, corrected habits and behaviour and also play a role in the psychological part of drug withdrawal processes.
The brain is obviously not as ultra-malleable as you claim, otherwise serial killers like the BTK killer would simply need to sit around a couple years for their cells to automatically be replaced and they'd no longer by a psychopathic, serial killer. Humans would also not have any discernable type of personality at all and would all just be interchangeable 'units'.