For the investor who has bet on a stock price going down, the hope is to be able to buy the shares back at a lower price than the original short was executed at. There is no timetable for the short investor to follow, so they can wait as long as they wish to repurchase the shares. However, if the stock begins to rise above the price the shares were shorted at, the investors' broker may require them to execute a buy to cover order as part of a margin call. To prevent this from happening, investors should always keep enough buying power in their account to make a "buy to cover" trade based the current market price of the stock.