I left a good 5 inches on my Bierre de Garde in my 15 gallon mini-brew conical recently. It wasn't enough. The trappist yeast actually lifted the lid a good inch off the fermenter (I don't clamp it down. I think this was partly due to the large starter I made and the pure O2 I oxygenated with. I'll probably hook up a blow off tube on the Tripple I'm going to toss onto the cake in another week.
Drinking on the keg: BPA, Brown Ale, Dry Mead, Wee Heavy aged on Oak, CAP
Drinking in the Bottle: