So, if we figured that there were 5 high-tech civilizations in our galaxy at any given moment...the nearest could easily be tens of thousands of light-years away.
Even if we assume the (imo, highly optimistic) 5000 technological civilizations, the nearest could be dozens or hundreds of light years away.
SETI's lack of success would probably argue that the more optimistic equation values are probably too high, and the nearest civilization (if there is one) is more than a thousand light years away. Long way to come visit.
Now, let's consider ways and means... currently we have little theoretical basis for faster-than-light travel, mostly just some speculation based on hypothesis considered "fringe" by the scientific establishment. Yet, as someone noted, we are probably vastly more ignorant of the true physical laws of the universe than we think we are. Note how the extrasolar planet search has revealed results drastically different from what the scientific establishment expected.
So...there's relativistic (near-c) space travel, using theoretical time dilation to make the journey seem livable. Approaching c (light speed) using reaction drives systems we can conceive of but not yet build, would be very expensive for even a very small payload. There's also the cosmic ray and micrometeor problems, due to blue-shift and ultra-velocity impacts respectively. Unmanned seems more likely than manned, barring some kind of reactionless drive system and advanced navigational shield.
There's also the slow-travel "ark" or "cold sleep" concepts, with decades or centuries passing in transit, and travellers either in suspended animation, or else living and dying by generations during the transit. Still expensive and difficult, but the possibility of live visitors exists.
Only with the relativistic ship would a civilization likely travel more than a few dozen light years from home.
Now, FTL (faster than light)... no real theoretical basis, more like scientific speculations such as the Alcubierre Warp Drive (a mathmatical construct which could theoretically exceed lightspeed, but the energy requirements are enormous). Even more speculatively, wormholes, hyperspace, etc. What the "speeds" and limitations of such an FTL drive might be, we could not know and speculation is probably useless.
SO, we probably only get visitors in sublight ships if they are pretty close by, less than a hundred light years...and SETI results suggest that is unlikely. Even then, a giant fleet with invading armies is improbable given the massive resources involved in such an effort.
Of course, extremely advanced aliens might not need a huge army to exterminate us... but would aliens necessarily be that much more advanced?
...con't....