No, it would not be an instant reaction, though nice try on her part. Every drug substance has a half life, this life can be extended with repetitive long term use, and as such it would actually take long for the drug to start moving out of their system for them to have withdrawals.
I just looked it up, and with normal, short term use, you are looking at two or three days before the amount of the drug significantly drops in someone's system, so with long term use, you would be looking at a week or so.
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