I want to say around 60 (64 I think?). Only pulled it off a couple of times though, and it was always on Refinery. Refinery is one of the best to play on as a marine in my opinion. You hole up with a team in the corridors, incoming enemies have nowhere to hide. That, and couple it with the size of the stage, and add the aliens and predators getting into arm wrestling matches that you can interrupt with a few bursts of the pulse rifle... I was so proud those few times I did that (hell, I'm always stoked when I break 50, which doesn't happen often either). The games I did that in were also full games, so it made it easier than you had it.
I usually average around 30, give or take a few kills.
You're pretty badass at AvP. If you were on PC in a full match, I could see you breaking 70. And I agree with you totally on the stealth kills, and how awesome this game is. Love your videos, too.