I've never had a battery vent, so I don't know how long it takes. Depending upon the reason for it venting, I'm sure it varies. If you're paying attention and being wise, a faulty battery would probably be you're only reason for venting to occur, and I think you'd be able to unscrew the switch, or tube on the 4Nine and get away from the battery. There are many ways to vent a battery, though. A hard short might be a quick way to vent, I'm sure someone here has the unfortunate experience.
If you have a 4nine, look over it and you'll see it has no venting at all. The two holes in the top cap are for aid in unscrewing it, when an atty is attached, they don't function as vents, and the hole in the switch has glue to hold the magnet. It's hybrid connection makes it a mod that requires attention to be paid to what atty you use more than others. If the 510 doesn't protrude far enough from the threading and you make contact with the pin and the threading, you have a short. Also, I think all clones, and all newer authentics should have a delrin insulator on the inside of the top cap, but the original authentics did not. Flat top batteries and no delrin is not good, either.
I love my 4Nine, but I have a few attys I won't use with it, and my VTC5's auto fire everything, so I use LG's with it. I've never used an Atlantis, but what
@Hartoled said is smart advice. I wouldn't dare put a spring loaded 510 pin atty on it.
If you want to have venting, a dremel and some grinding on the top cap from the holes to the edge will help. Again, great mod, but one you need to pay attention to, and preferably have some experience. I respect all my mechs, especially the 4Nine.