no stock lending, page-11

My understanding is there is no time limit per se other than what makes commercial sense for the short seller in the trade.
I found the article: http://www.theaustralian.news.com.au/story/0,25197,23364916-643,00.html

but to paraphrase the relevent section...
When stock becomes difficult to borrow, the hedge funds can simply "wear" the ASX fail fees, which range between $500 a day and $2000 a day. In many cases this is cheaper than borrowing stock to short sell a company.

As one broker said, if seller A sells 100,000 shares at $5 and fails to deliver, the ASX charges it a maximum fine of $2000. If the company is subject to rumours and other tactics and falls to $3 a share, after 10 days the stock is bought back. The profit is $200,000 and the fail fees are $20,000, which goes to the ASX.