It's not quite any time - there are some rules that need to be adhered to, but there is a great deal of flexibility as to when an election can be called, and if a party loses an election, there is no real transition period - the newly elected government takes office almost immediately. So, yes, it is important that the opposition is ready to form government immediately.
Back in 1975, a Consitutional crisis lead the Governor-General to dismiss the entire government from office and appoint the Opposition to replace them. The Opposition is expected to be ready for such a situation to occur, evn though it's very rare.
Could not have explained it as succinctly as yourself - the terms "Liberal", "Labor", "Democrat" have different meanings in different democracies, it comes down to "Right", "Centre", "Left" in any interpretation.
You forgot to mention the role of the Governor General in the removal of the ruling party in 1975. To our friends still trying to understand the hybrid Westminster/Australian government model, the Governor General is the Queen's representative in Australia (the Queen of the British Empire that is). The Governor General rarely gets involved, and is the only statesman with the authority to remove the current Prime Minister from office without an election of the people, under the direction of the Queen.
When this happenned in 1975 it was rumoured that the CIA was involved. The proof is inconclusive.....