How
long it takes you to write any speech usually depends on what your
opinions on the topic are.
If you feel very strongly and have lots to
stay, it's usually very easy to put all of your ideas down on paper
and then make cuts so that you only have the most relevant and/or
interesting information.
If
the topic is something that you don't have strong opinions on, or
perhaps you don't know a great deal about, then writing a speech of
any length can prove a lot more difficult and time consuming. You may
have to put more time into research and familiarising yourself with
the topic.
With
your speech, "When does a migrant become Australian", it shouldn't be
too difficult as it's not really something that you need facts and
figures for, it's more a matter of opinion, so you are free to put
your own ideas and thoughts into the content.