I got a problem with races or in that case the flood coming from a other galaxy.
Ok i read that in "The fall of reach" (i´m still waiting for my copy ) is sad that
the time traveling from earth to reach took half a day. While reach was 10 light years
away from earth.
So you could travel 20 ly in one day. (i´m ignoring the fact right now that the covenant
can travel slightly faster than the humans in slipspace and that there is a 5% to 10%
difference in travel-time each time).

If you just want to go from one end to the other end of the galaxy it would take you about
5000 Days! (the galaxy is about 100,000 ly )
The nearest galaxy is Andromeda witch is 2 million ly away. Just to get there would take
you about 273 years.

The Forerunners wanted to starve the Flood to death... the traveling time could have done
that for them

yeah, i haven't been sure how to approach the astronomy suggestions in the halo universe. on one hand i don't want to ruin the story by holding the writers to the same standards of an astronomy paper, but on the other hand they cover a LOT of their bases well, so why not all of them?

the distance from here to the center of our galaxy is HUUUUUUGE. but it is nothing compared to the distances BETWEEN galaxies.

the idea of a galactic empire stretches the brain enough, multi-galactic empires (or even just coordinated presences) is verging on silly.

My guess is that the day of traveling was between slipspace and the planet. I would guess that the time in slipspace is instantaneous, and that the ships simply need to get far enough from planets not to cause damage when they enter slipspace. That's what takes the day.

From Halopedia:
Faster-than-light travel is not instantaneous; "short" jumps routinely take up to two months, and "long" jumps can last six months or more, which is why most UNSC ships have cryo chambers.

http://halo.wikia.com/wiki/Slipspace