It can take as little as 2 weeks to learn a full QB to sufficient level to pass. Not a wise move though. If you think you'll continue to read and learn the ATPL text books post exam, you are fooling no one but yourself.
Many do employ this tactic, you'd be surprised at the FOs who turn up on-line without some pretty basic knowledge.
A famous Old Boy once turned up at my school assembley and said, "half of everything you learn at school is completely useless and is pointless learning". All the teachers turned round looking aghast, until he said "but you don't know which half, so study it all". Very true.