This really is one of those 'how long is a piece of string' questions, to which there can be no definite answer.
Extradition requires a request at inter-governmental level. The Crown Prosecution Service (or other relevant body, such as the Serious Fraud Office) has to persuade the Home Office that such a request is necessary. The F.O. will then submit the request via the British Embassy (or High Commission) in the relevant country. The receiving country (Australia in this case) then has to decide whether the request for extradition meets all of the necessary legal requirements. In very straightforward cases all of those criteria can be met within a few days. In other cases the procedures can take many months.
Then the alleged offender has to decide whether to accept extradition, or to challenge it. There have been some cases, reported in the press, where a suspected offender has been extradited back to the UK within a few days of him arriving in a foreign country, because he has decided not to fight extradition. However there are also several ongoing cases of attempted extradition (in either direction) where the legal challenges have been going on for several years. (The USA have been attempting to extradite Gary McKinnon from the UK for the past 5 years. Such a delay is far from unusual).
Because of the potential massive legal costs involved in seeking extradition, the Home Office would be unlikely to consider it except for the most serious of (alleged) offences. In 2005 (which is the most recent year for which I can find statistics), not a single person was extradited from Australia to the UK.
Chris