My mom's labors cut in half each time - 5 hours, 2.5 hours, 1.5 hours (that was me
). I was almost born in the parking lot.
My first was 2.5 hours, but my water broke 4 days earlier and the baby was only 3 lbs 13.4 oz (29w4d). That's measuring from first contraction to hearing baby cries. I don't expect my second labor to be faster than that, but I wouldn't be surprised if it were similar timeframe, barring weird positioning or something. But that's just how my family labors. My sister had similar first time labor time as what my mom had (she hasn't had a second, and probably won't).
I imagine the average 6-8 hours is kind of like the average for first time moms being 14 hours. If you were above or below the average the first time, you might be above or below the average the second time too (but as PPs mentioned, anything can happen and you can have some wildly long or short labor that doesn't match your other labors!).
I can tell you in November how my 2nd turns out. At least, I HOPE it's November, and not August.