(Might need to check the ATO site to see if the limit has...

(Might need to check the ATO site to see if the limit has risen)

Claiming > $5k franking credits - Hold > 45 days
Claiming < $5k franking credits - Hold however long you want.

Actually this is almost correct. From here http://www.ato.gov.au/corporate/content.asp?doc=/content/18898.htm

"The 45-day rule aims to eliminate franking credit trading where franking benefits are received by someone other than the true economic owner of the underlying shares. The rule requires resident taxpayers to hold shares for at least 45 days to be eligible to receive franking benefits from dividends paid on shares. Furthermore, even if the shares were held for at least 45 days, the franking credit is denied if the resident taxpayer has eliminated 70 per cent or more of the ownership risk through other financial transactions during that period. Hence, the rule also specifies a 30 per cent minimum level of ownership risk.

The 45-day rule applies to direct and indirect holdings. However, special considerations apply in relation to interests in shares held through trusts where the interest is itself inherently risk-free.

There are several exceptions to the 45-day rule. One of these is the small shareholder exemption which exempts shareholders from applying the 45-day rule by electing to limit the amount of total franking rebates to which they are entitled. The threshold for the full franking rebate under this rule was $2000." (Changes have made this now $5000)