You can keep a short position for a long time, as long as you can maintain margin, which gets bigger if the stock price continues increasing, and pay margin interest - there is no set date when the short has to he closed, it's indefinite. Sometimes the lender who loaned you the stock can ask for it back, and if you can't locate any more shares to borrow to replace the returned shares, you might be forced to buy the shares back and close the short, but this is not common, at least during normal market conditions.