In terms of average speed, its very fast. Hundreds of light years an hour
Apparently (according to James Gunn) GotG takes place in the Andromeda galaxy. Andromeda is approximately 220,000 light years in diameter. So taking the quote:
Halfway across the quadrant
Lets assume that means literally one-eighth of a circle, at some distance out from the center of the galaxy. Let's assume a distance literally one eighth of a circle one half the diameter of the galaxy.
I won't clutter the answer with the math, but this is a distance of approximately 43,000 light years.
We don't know exactly from the movie how long they have been traveling. I think it's only meant to be a few days. So assuming 3 days, they have travelled on average around 600 light years an hour.
However it appears that they do this by jumping between locations
Now as we see in Guardians of the Galaxy Vol 2, their movement isn't simple speed as such. They are making "jumps" across the larger distances. These appear to be practically instantaneous so pointless to quantify in terms of speed.
[Note the language "halfway across the quadrant" is so vague, the numbers could be defined in many different ways ... half the radius of the galaxy for example would be halfway across a quadrant, and would be quite a bit more than 43,000 light years. The point of the answer is .... its very fast, but not a steady speed because they jump]