Hi,

I would say that the length would depend on many variables, for example, resources, forces available, preparations, the population's willingness and technology - like how people travel around the galaxy - and are you conquering or colonizing? In this case, the galaxy was already populated so if you can keep the people from revolting you can pick up resources and manpower as you go. 

@Jochern, Interesting answer, and I assume Star Wars also assumes a well populated galaxy. After thinking off and on about how I'd go about settling a galaxy, perhaps I've limited my thinking.