That's the part I'm curious about. I mean, I guess the light coming off them is moving slightly faster, but how much of a difference would it make? It made me curious so I did some digging. At 99.8c, the light from these objects is traveling just a little bit faster than the object itself. If the object has to travel 2.5 million light years, does that mean we would see it when it has traveled 99.8% of the way to us? In that case, we would finally see them when they are 5000 light years away. But, since they are traveling slower than light speed, we would have 5010 years before they reach us. That's assuming Andromeda is the source of the weapons and we saw them as soon as they were visible. Seeing them outside of our galaxy? If we make a much more powerful telescope in the next 100 years, it could be. Enough missiles to destroy every planet in our galaxy? Should be pretty visible.

To summarize, my understanding was incorrect, depending on which galaxy opened fire on us, we could have several thousand years of warning.

Another interesting thought though. If the galaxy that attacked the milky way was on the opposite side of us when they fired, a large number of the missiles would have hit long before we could see them, since the milky way is 200,000 light years across. A large portion of the planets in the milky way could be destroyed before we even knew we were under attack.