We actually added the gelatin to the secondary as we would priming sugar to a bottling bucket, then just racked on top of it while it was still hot. It really only takes a couple days and sometimes, in our case, even less.
bhatchable, 4.5 gallons were put into the fermenter (damn pumpkin in the mash held on to a lot of liquid) and there is currently ~4.25 gallons in the secondary with a super compact cake < 1cm thick at the bottom. My "expert opinion", which is based off of absolutely nothing, would be you'd get 7.5 gallons out of your 8 gallons. But really, that's just a rough estimation