Something else to consider is the "buy-in." Remember that you borrowed 100 shares from a margin account. If the client of this account decides to sell the shares, then the broker will need to get a borrow from someone else. If this is not possible, your broker has the right to force you to cover your short position. Sounds kind of harsh, huh? Well, when you sign a margin account, there is a clause in the agreement that gives this powerful right to your broker.
Most of the time, you can hold a short for as long as you want. You can, however, be forced to cover if the lender wants back the stock you borrowed. They can't sell what they don't have, and so your brokerage will have to either come up with new shares to borrow, or you'll have to cover. This is known as being "called away." It doesn't happen often, but is possible if many investors are selling a particular security short.
I've always been puzzled with folks saying I'll short to "0". Personally, I never short for a long time. Anything between -15 to +20% is already pretty good in my book.
