The most common argument is that, even if it takes 10000 years to travel to the nearest star, a race could make the trip, set down a colony, and then build new star ships to send to the 2 or 3 stars nearest their new home. Even if it took 100 years to create the necessary industries to do this at the new colony, by repeating this process over and over again with each new colony entering the effort every 10,000 years, within 10 to 50 million years, much of the Milky Way would get colonized because of the exponential growth of new colonies added to the effort. So, if it only takes 10 to 50 million years to colonize the galaxy, where are the traces of these other civilizations? We have yet to find any indication we have been visited, so there must be something faulty with the logic. The most obvious error is that under any circumstances, interstellar travel is 'easy' but time consuming. The other error is that an intelligent civilization would undertake such a task when there are simpler ways to study the universe using telescopes or unmanned, automated probes.
Whatever the answer, we had better start seriously considering such journeys ourselves, because in 500 to 700 million years the Earth will be totally uninhabitable as the Sun continues to brighten and our oceans begin to evaporate.