Did they consider new impacts?

Would it take a trillion years to spread life through a volume of space the size of our galaxy?

Gliese 581 is located 20.3 light years from Earth, so it would have taken the rocks about a million years to make the journey.

But what if life did take hold and cover a planet and then 100 million years rock hits it spreading life 20 l.y. onward for another million year journey?

The milky way is about 100,000 l.y. across. So that would be about 5000 jumps. Meanwhile some from the first jumps would continue outward up to 800 l.y. at a time. That would only be 125 jumps.

So when do we get to meet them?