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Unless your broker needs those shares back or you lose your margin to continue holding that short then you can hold shorts indefinitely. That said you're paying interest holding shorts so if you hold it too long then it could be an expensive proposition. The other thing I wanna point out is stocks in general are much harder to short than currencies or future contracts. Stocks can't be shorted if too much people are short already (25% of the float in the US I am not sure with other countries), in the currency market you can short forever without problems. (Been short USD for a couple of years now something I won't be able to do in stocks)
Right, I remember the three of us (you, me, & Rick Ong) tried to short NEWC then we all found out at the same time, there are no more shares to short.