I'll confess that speed-of-light physics are far from my strength, but the way I understand it it's certainly possible to travel at at least half-light speeds (you would not experience any significant adverse effects from space or time going weird around you).

That means that even for a species with a life span similar to that of humans, but possessing advanced enough technology to travel at half-light speed, it would not be impossible to launch an invasion from a point in space about 10 to 15 ish light years from here. If they started now, they'd be here in 30 years. The question of whether this is a realistic prospect comes down to how important it is to an intelligent species to have access to a planet that demonstrably can sustain life. My guess would be- really damn important. Important enough that we can be fairly sure that we ourselves will definitely look for that sort of planet in that sort of distance and time-frame, once we have achieved the technological capacity to do so.

And, like I say, the above is assuming a species with a life span similar to ours. Which is a big assumption. Maybe my hypothetical space invaders will have a typical lifespan of 1000 years, so that 50 years are nothing for them - a 20th of their lifetime. 5 of our years. Would we launch a 5-year space invasion to take control of a planet capable of harboring life? I bet we would. We'd send in the space marines alright (because an inhabited planet would present that sort of challenge to our control).

And who knows- if an alien civilisation is advanced enough to travel at half the speed of light, then maybe they also have the technology to artificially expand their life span. Maybe 1000 years is 20% of their life time. Maybe it's 1%. Maybe they don't really need to die anymore? So they can travel anywhere they like, at their own time. Of course in that case they can just wait around a few more centuries, until we've wiped ourselves out- problem solved. But maybe they have other reasons than their life span to hurry and take over a nice little blue gem like ours (theirs is all but exhausted, or a Tyranid Hive Fleet is approaching, that sort of thing).

What I'm saying is that the vastness of the distances across interstellar space may be a big problem for us but that doesn't necessarily mean that it's going to be as big a problem for a much more technologically advanced civilisation, also. And maybe we currently don't have the motivation to attempt such travel even if we had the means- but that, too, is not necessarily a quality shared with every other technological civilisation in our near galactic neighbourhood.