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When do I need to return short stock to the lender
If the owner of the stock wants it back, they "call" it back. There are no guarantees of how long you can keep it for your short, or the cost involved to hold it.  Usually, everyone knows about a particular set-up (e.g. a warrant or convertible bond mispricing) that is attractive for arbitrage. This causes the associated stock to be in high demand thus expensive to borrow for shorting, or impossible to find for any price at all.